What does enctype='multipart/form-data' mean?

Question

What does enctype='multipart/form-data' mean in an HTML form and when should we use it?

https://www.w3.org/html/wg/spec/association-of-controls-and-forms.html#attr-fs-enctype — IEnjoyEatingVegetables, Aug 09 '18 at 10:26
It's for POSTing entire files as part of form submission. I thought it might have something to do with multiple parts of text-form input, but it doesn't, it's just for file uploads. — Janac Meena, May 07 '21 at 00:06

Quentin · Accepted Answer · 2020-04-23T11:37:34.367

When you make a POST request, you have to encode the data that forms the body of the request in some way.

HTML forms provide three methods of encoding.

application/x-www-form-urlencoded (the default)
multipart/form-data
text/plain

Work was being done on adding application/json, but that has been abandoned.

(Other encodings are possible with HTTP requests generated using other means than an HTML form submission. JSON is a common format for use with web services and some still use SOAP.)

The specifics of the formats don't matter to most developers. The important points are:

Never use text/plain.

When you are writing client-side code:

use multipart/form-data when your form includes any <input type="file"> elements
otherwise you can use multipart/form-data or application/x-www-form-urlencoded but application/x-www-form-urlencoded will be more efficient

When you are writing server-side code:

Use a prewritten form handling library

Most (such as Perl's CGI->param or the one exposed by PHP's $_POST superglobal) will take care of the differences for you. Don't bother trying to parse the raw input received by the server.

Sometimes you will find a library that can't handle both formats. Node.js's most popular library for handling form data is body-parser which cannot handle multipart requests (but has documentation which recommends some alternatives which can).

If you are writing (or debugging) a library for parsing or generating the raw data, then you need to start worrying about the format. You might also want to know about it for interest's sake.

application/x-www-form-urlencoded is more or less the same as a query string on the end of the URL.

multipart/form-data is significantly more complicated but it allows entire files to be included in the data. An example of the result can be found in the HTML 4 specification.

text/plain is introduced by HTML 5 and is useful only for debugging — from the spec: They are not reliably interpretable by computer — and I'd argue that the others combined with tools (like the Network Panel in the developer tools of most browsers) are better for that).

@Quentin Excuse me, what will be any probable problem if we use multipart for all forms? with and whit out files. — Webinan, Oct 20 '13 at 09:47
It doesn't make sense for GET forms, and it makes the file size of requests bigger. — Quentin, Oct 20 '13 at 10:46
@Quentin does multipart form data send as a stream by default? — Growler, Feb 13 '17 at 23:06
@MasterJoe because it can have multiple data items separated by boundaries, see [RFC 2046 section 5.1.1](https://tools.ietf.org/html/rfc2046#section-5.1.1). — Ruslan, Feb 02 '21 at 14:27

Ciro Santilli新疆棉花TRUMP BAN BAD · Answer 2 · 2020-05-07T10:37:18.953

when should we use it

Quentin's answer is right: use multipart/form-data if the form contains a file upload, and application/x-www-form-urlencoded otherwise, which is the default if you omit enctype.

I'm going to:

add some more HTML5 references
explain why he is right with a form submit example

HTML5 references

There are three possibilities for enctype:

application/x-www-form-urlencoded
multipart/form-data (spec points to RFC7578)
text/plain. This is "not reliably interpretable by computer", so it should never be used in production, and we will not look further into it.

How to generate the examples

Once you see an example of each method, it becomes obvious how they work, and when you should use each one.

You can produce examples using:

nc -l or an ECHO server: HTTP test server accepting GET/POST requests
an user agent like a browser or cURL

Save the form to a minimal .html file:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8"/>
  <title>upload</title>
</head>
<body>
<form action="http://localhost:8000" method="post" enctype="multipart/form-data">
  <p><input type="text" name="text1" value="text default">
  <p><input type="text" name="text2" value="a&#x03C9;b">
  <p><input type="file" name="file1">
  <p><input type="file" name="file2">
  <p><input type="file" name="file3">
  <p><button type="submit">Submit</button>
</form>
</body>
</html>

We set the default text value to aωb, which means aωb because ω is U+03C9, which are the bytes 61 CF 89 62 in UTF-8.

Create files to upload:

echo 'Content of a.txt.' > a.txt

echo '<!DOCTYPE html><title>Content of a.html.</title>' > a.html

# Binary file containing 4 bytes: 'a', 1, 2 and 'b'.
printf 'a\xCF\x89b' > binary

Run our little echo server:

while true; do printf '' | nc -l 8000 localhost; done

Open the HTML on your browser, select the files and click on submit and check the terminal.

nc prints the request received.

Tested on: Ubuntu 14.04.3, nc BSD 1.105, Firefox 40.

multipart/form-data

Firefox sent:

POST / HTTP/1.1
[[ Less interesting headers ... ]]
Content-Type: multipart/form-data; boundary=---------------------------735323031399963166993862150
Content-Length: 834

-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="text1"

text default
-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="text2"

aωb
-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="file1"; filename="a.txt"
Content-Type: text/plain

Content of a.txt.

-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="file2"; filename="a.html"
Content-Type: text/html

<!DOCTYPE html><title>Content of a.html.</title>

-----------------------------735323031399963166993862150
Content-Disposition: form-data; name="file3"; filename="binary"
Content-Type: application/octet-stream

aωb
-----------------------------735323031399963166993862150--

For the binary file and text field, the bytes 61 CF 89 62 (aωb in UTF-8) are sent literally. You could verify that with nc -l localhost 8000 | hd, which says that the bytes:

61 CF 89 62

were sent (61 == 'a' and 62 == 'b').

Therefore it is clear that:

Content-Type: multipart/form-data; boundary=---------------------------735323031399963166993862150 sets the content type to multipart/form-data and says that the fields are separated by the given boundary string.

But note that the:
```
boundary=---------------------------735323031399963166993862150
```
has two less dadhes -- than the actual barrier
```
-----------------------------735323031399963166993862150
```
This is because the standard requires the boundary to start with two dashes --. The other dashes appear to be just how Firefox chose to implement the arbitrary boundary. RFC 7578 clearly mentions that those two leading dashes -- are required:

4.1. "Boundary" Parameter of multipart/form-data

As with other multipart types, the parts are delimited with a boundary delimiter, constructed using CRLF, "--", and the value of the "boundary" parameter.
every field gets some sub headers before its data: Content-Disposition: form-data;, the field name, the filename, followed by the data.

The server reads the data until the next boundary string. The browser must choose a boundary that will not appear in any of the fields, so this is why the boundary may vary between requests.

Because we have the unique boundary, no encoding of the data is necessary: binary data is sent as is.

TODO: what is the optimal boundary size (log(N) I bet), and name / running time of the algorithm that finds it? Asked at: https://cs.stackexchange.com/questions/39687/find-the-shortest-sequence-that-is-not-a-sub-sequence-of-a-set-of-sequences
Content-Type is automatically determined by the browser.

How it is determined exactly was asked at: How is mime type of an uploaded file determined by browser?

application/x-www-form-urlencoded

Now change the enctype to application/x-www-form-urlencoded, reload the browser, and resubmit.

Firefox sent:

POST / HTTP/1.1
[[ Less interesting headers ... ]]
Content-Type: application/x-www-form-urlencoded
Content-Length: 51

text1=text+default&text2=a%CF%89b&file1=a.txt&file2=a.html&file3=binary

Clearly the file data was not sent, only the basenames. So this cannot be used for files.

As for the text field, we see that usual printable characters like a and b were sent in one byte, while non-printable ones like 0xCF and 0x89 took up 3 bytes each: %CF%89!

Comparison

File uploads often contain lots of non-printable characters (e.g. images), while text forms almost never do.

From the examples we have seen that:

multipart/form-data: adds a few bytes of boundary overhead to the message, and must spend some time calculating it, but sends each byte in one byte.
application/x-www-form-urlencoded: has a single byte boundary per field (&), but adds a linear overhead factor of 3x for every non-printable character.

Therefore, even if we could send files with application/x-www-form-urlencoded, we wouldn't want to, because it is so inefficient.

But for printable characters found in text fields, it does not matter and generates less overhead, so we just use it.

Great post. Question: Why do we have 3 bytes for each non printable character? For eg: for unicode U+03C9, we have %CF%89: this is 2 extra bytes for the two "%". Is my understanding correct? — Khanna111, Aug 06 '15 at 18:42
@Khanna111`%CF` is 3 bytes long: `%`, `C` and `F` :-) Story of making it human readable. — Ciro Santilli新疆棉花TRUMP BAN BAD, Aug 06 '15 at 18:50
I had to use a slightly different listener: `while true; do printf '' | nc -lp 8000; done` — starfry, Aug 27 '15 at 18:36
@starfry what error did you get on the command I've given? What is your `nc` version and distro? I'm `netcat-openbsd` 1.105-7ubuntu1 Ubuntu 14.04.3. — Ciro Santilli新疆棉花TRUMP BAN BAD, Aug 28 '15 at 08:05
I have **gnu-netcat 0.7.1-5** on **Arch Linux**. It's doc states that `-l` is needed for listen mode and `-p` to specify the port. The optional host and port arguments restrict connections to a specific remote host and port. There is no error because the command as written in the answer opens a connection that listens on a random port for connections from `localhost:8000`. — starfry, Aug 28 '15 at 14:52
@starfry can you check that: `while true; do printf '' | nc -lp 8000 localhost; done` works for you? It seems to work on both version, if it does I'll update to use that instead, otherwise I'll just mention both commands. — Ciro Santilli新疆棉花TRUMP BAN BAD, Aug 28 '15 at 15:41
Yes, `while true; do printf '' | nc -lp 8000 localhost; done` works fine. — starfry, Aug 28 '15 at 15:44
+1, very comprehensive. You may want to align the boundary string under "Therefore it is clear that:" with the one in the request — Saar, Mar 26 '17 at 03:14
On OS X, `nc` won't accept both the `-l` and the `-p` arguments simultaneously. But this works for me: `while true; do printf '' | nc -l 8000; done`. — PhilipS, Apr 25 '17 at 02:03
@PhilipS actually, `-p` is unecessary on Linux as well and I've removed it. Thanks for bringing that up. — Ciro Santilli新疆棉花TRUMP BAN BAD, Apr 25 '17 at 05:37
A small but important point that isn't mentioned is that the boundary specified in the `Content-Type` has two hyphens (`--`) less, i.e. when actually using the boundary in the message body, you must prefix it with `--`. Also, the last boundary must be suffixed with `--`, but that is easy enough to notice. See https://stackoverflow.com/questions/3508252/what-is-the-in-multipart-form-data — Bernard, Jul 03 '17 at 12:34
For those who triy to construct that file upload message it's still not clear where/what new line chars need to be added — Pavel P, Jun 05 '19 at 22:33
What is the purpose of putting so many dashes in the boundaries? Is it for easy visual recognition? — Daniel says Reinstate Monica, Oct 09 '19 at 16:10
@DanielPendergast not sure why this default. In general, to not conflict with the same number of dashes present in content of fields which would lead to ambiguity. — Ciro Santilli新疆棉花TRUMP BAN BAD, Oct 09 '19 at 16:11
So far as I can tell, the point of putting ANY DASHES AT ALL in the boundary is to make it impossible to check the syntax of the request by eye. Please don't use them in your boundary tokens. — Dewi Morgan, Nov 11 '19 at 17:01
@DewiMorgan You are completely right. I edited the post and removed the dashes from the boundary string. — Max, May 07 '20 at 10:11
@Max I rejected temporarily, what's the Firefox version you tested on? — Ciro Santilli新疆棉花TRUMP BAN BAD, May 07 '20 at 10:12
@CiroSantilli冠状病毒审查六四事件法轮功 The point is not that dashes in the boundary string wouldn't work. They do work perfectly. But, as Dewi Morgan said: They are unnecessary and highly confusing because the multipart/form-data encoding requires setting "--" before each boundary and after the last boundary. — Max, May 07 '20 at 10:18
@Max I understand. But I would like to stick to a concrete example from an specific browser in the answer. We should definitely mention the extra `--` thing on the answer though. — Ciro Santilli新疆棉花TRUMP BAN BAD, May 07 '20 at 10:22
@CiroSantilli郝海东冠状病六四事件法轮功 - why is it called "multipart" ? — MasterJoe, Sep 01 '20 at 22:37
@MasterJoe this is explained on the book "The art of web development" by John Debber, please go and read it. Just kidding! I guess it's because each `boundary=` forms a "part". — Ciro Santilli新疆棉花TRUMP BAN BAD, Sep 02 '20 at 06:35
@CiroSantilli郝海东冠状病六四事件法轮功 - haha. did you read my profile ? — MasterJoe, Sep 02 '20 at 06:37
here is not a place for say tnx but this answer is the one that make me deep understanding which I love — kia nasirzadeh, Oct 17 '20 at 09:40

score 99 · Answer 3 · edited Dec 12 '12 at 11:30

99

enctype='multipart/form-data is an encoding type that allows files to be sent through a POST. Quite simply, without this encoding the files cannot be sent through POST.

If you want to allow a user to upload a file via a form, you must use this enctype.

edited Dec 12 '12 at 11:30

Mohammed H

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answered Dec 24 '10 at 12:49

Matt Asbury

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So.. if the file is not a binary file then can we work without this ? – Yugal Jindle Aug 27 '13 at 00:01
From what I understand, you can use `multipart/form-data` for sending non-binary files but it is inefficient. I believe using `application/x-www-form-urlencoded` is the correct way to send non-binary data but someone with more experience with non-binary files may need to correct me. – Matt Asbury Aug 27 '13 at 09:36
13

The main advantage to using `multipart/form-data` for sending a file is that it will work automatically in both frontend and backend. You don't have to do any special handling. All files are binary even if they should only contain text. `application/x-www-form-urlencoded` is the standard way to POST a form without attached files. `multipart/form-data` is the standard way to POST a form with attached file(s). (There are also numerous other encodings, such as `application/json` and `application/json-patch+json`, which are common for communication between server and client.) – Daniel Luna Sep 19 '13 at 17:34
6

Its worth pointing out you can base64 encode your image and send it as plain string data . – Heavy Gray Jul 14 '14 at 22:46
5

Further to @Prospero's comment above: you can absolutely send files via POST without using `multipart/form-data`. What you *can't* do is do that using an ordinary HTML form submission, without JavaScript. Setting a form to use `multipart/form-data` is the only mechanism that *HTML* provides to let you POST files without using JavaScript. I feel like this isn't clear enough in the answer, and that a naive reader might think that the inability to send files without `multipart/form-data` is a limitation of *HTTP*; that's not the case. – Mark Amery Feb 02 '19 at 14:29

Andry · Answer 4 · 2020-04-12T17:25:06.007

When submitting a form, you tell your browser to send, via the HTTP protocol, a message on the network, properly enveloped in a TCP/IP protocol message structure. An HTML page has a way to send data to the server: by using <form>s.

When a form is submitted, an HTTP Request is created and sent to the server, the message will contain the field names in the form and the values filled in by the user. This transmission can happen with POST or GET HTTP methods.

POST tells your browser to build an HTTP message and put all content in the body of the message (a very useful way of doing things, more safe and also flexible).
GET will submit the form data in the querystring. It has some constraints about data representation and length.

Stating how to send your form to the server

Attribute enctype has sense only when using POST method. When specified, it instructs the browser to send the form by encoding its content in a specific way. From MDN - Form enctype:

When the value of the method attribute is post, enctype is the MIME type of content that is used to submit the form to the server.

application/x-www-form-urlencoded: This is the default. When the form is sent, all names and values are collected and URL Encoding is performed on the final string.
multipart/form-data: Characters are NOT encoded. This is important when the form has a file upload control. You want to send the file binary and this ensures that bitstream is not altered.
text/plain: Spaces get converted, but no more encoding is performed.

Security

When submitting forms, some security concerns can arise as stated in RFC 7578 Section 7: Multipart form data - Security considerations:

All form-processing software should treat user supplied form-data
with sensitivity, as it often contains confidential or personally
identifying information. There is widespread use of form "auto-fill" features in web browsers; these might be used to trick users to
unknowingly send confidential information when completing otherwise
innocuous tasks. multipart/form-data does not supply any features
for checking integrity, ensuring confidentiality, avoiding user
confusion, or other security features; those concerns must be
addressed by the form-filling and form-data-interpreting applications.

Applications that receive forms and process them must be careful not to supply data back to the requesting form-processing site that was not intended to be sent.

It is important when interpreting the filename of the Content-
Disposition header field to not inadvertently overwrite files in the
recipient's file space.

This concerns you if you are a developer and your server will process forms submitted by users which might end up containing sensitive information.

The stuff about security after the most recent edit is all irrelevant to the question of what `enctype`s do. I know it's literally from the `multipart/form-data` RFC, but nonetheless it's an arbitrary dump of security considerations about submitting forms that are entirely orthogonal to whether data is sent as `application/x-www-form-urlencoded` or `multipart/form-data`. — Mark Amery, Feb 03 '19 at 09:51

score 45 · Answer 5 · edited Oct 13 '14 at 21:09

45

enctype='multipart/form-data' means that no characters will be encoded. that is why this type is used while uploading files to server.
So multipart/form-data is used when a form requires binary data, like the contents of a file, to be uploaded

edited Oct 13 '14 at 21:09

Volker E.

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answered Jul 04 '13 at 09:13

GP Singh

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score 9 · Answer 6 · answered Sep 25 '13 at 11:53

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Set the method attribute to POST because file content can't be put inside a URL parameter using a form.

Set the value of enctype to multipart/form-data because the data will be split into multiple parts, one for each file plus one for the text of the form body that may be sent with them.

answered Sep 25 '13 at 11:53

sandy

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This implies that `POST` is likely to be sufficient for submitting a file via a form and that adding `multipart/form-data` is just a bonus in some vague way. That's not the case. Most files will absolutely require using `multipart/form-data`. – underscore_d Jun 11 '17 at 12:33

Premraj · Answer 7 · 2019-02-03T04:00:37.080

2

enctype(ENCode TYPE) attribute specifies how the form-data should be encoded when submitting it to the server.
multipart/form-data is one of the value of enctype attribute, which is used in form element that have a file upload. multi-part means form data divides into multiple parts and send to server.

edited Feb 03 '19 at 04:00

answered Dec 27 '15 at 01:29

Premraj

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I believe **enctype** does not stand for encryption type. There is no encryption involved at this level. My guess is either encoding type or enclosed type. But surely it is not encryption type. – Yeo Mar 03 '16 at 11:00
1

Your final bullet point here about `` and `` is irrelevant and confusing. – Mark Amery Feb 02 '19 at 14:55

score 1 · Answer 8 · answered Mar 10 '16 at 09:29

Usually this is when you have a POST form which needs to take a file upload as data... this will tell the server how it will encode the data transferred, in such case it won't get encoded because it will just transfer and upload the files to the server, Like for example when uploading an image or a pdf

score -2 · Answer 9 · edited Nov 14 '18 at 02:25

-2

The enctype attribute specifies how the form-data should be encoded when submitting it to the server.

The enctype attribute can be used only if method="post".

No characters are encoded. This value is required when you are using forms that have a file upload control

From W3Schools

edited Nov 14 '18 at 02:25

Pang

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answered Nov 11 '18 at 22:52

Rishad

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This quote doesn't even mention `multipart/form-data`. It's also pretty unclear; what does the sentence "No characters are encoded" even mean? -1. – Mark Amery Feb 02 '19 at 14:22