According to the MIME that you've given, you're reading data that's in non-textual binary format. You cannot convert it with iconv
or similar, because it's meant for converting text from one (textual) encoding to another. If your data is not textual, then a conversion to any character encoding is meaningless and will just corrupt the data, but not make it any more readable.
The typical way to present binary as readable text for inspection is hex dump. There's an existing answer for implementing it in c++: https://stackoverflow.com/a/16804835/2079303