Add access to additional multipart headers

[deathgrindfreak]

So I need some additional headers from multipart data (specifically a generated Content-MD5 header) and was wondering if you guys would be alright with a pull request adding this feature. I'm thinking it could just be passed in as an optional parameter in the 'field' and 'file' listeners, since that really shouldn't break backwards compatibility (though I haven't looked into the urlencoded type, my proposed change would really only impact the multipart type).

[mike-marcacci]

Hmm, how would this work? Generating a hash requires consuming the entire contents of the upload, which would result in a finished (read useless) stream being passed in the file event.

[mscdex]

'Content-MD5' is not a typical header supplied during form submission. You would need to use the underlying dicer library which is a more generic multipart parser.

[deathgrindfreak]

Yeah that's what I'm getting at. I'd basically like to be able to have access to additional headers parsed by dicer (they're just skipped for now I believe). So if I added an 'X-My-Customer-Header' for each part, I'd be able to access it in the file handler. I generate the hashes on the client side, so it wouldn't be something busboy would handle.