Add handling for multipart/mixed

[mikedilger]

This is where a single multipart section is itself a multipart section. This happens when a file input field is filled with multiple uploaded files.

[SkylerLipthay]

It's worth noting that this style is not seen in modern browsers, but I believe it should still be supported.

I'm not finding concrete specs for multipart/mixed. Is it historically only used to specify a named array of files? Or can you mix and match files and inline text content? Is it valid to infinitely nest multipart/mixed parts? Once I know what this library should support, I can implement this feature.

[mikedilger]

Interesting. Since it's not seen in modern browsers, I will mark this P-Low. Still useful just in case.

multipart/mixed is defined in RFC 1341 section 7.2. It parses very much the same as multipart/form-data parsed, so significant sharing of code would be expected.

The top level multipart/form-data section would specify in its Content-Disposition header the input name of the file input, so all the parts of any multipart/mixed should be considered as files submitted under that form input. Each one should offer a Content-Disposition header itself with a filename parameter, and if not, we may need some algorithm to make up filenames like "userfile1"..."userfileN" or whatever. (edited: just leave filename: None)

I personally wouldn't bother coding for the case of recursive descent, but you can if you like, I wouldn't consider it wrong.