Infer MultipartFile contentType from file type

[jamal-o]

Is there a timeframe for when this would be implemented? If not, I'd like to give it a go.

http/pkgs/http/lib/src/multipart_file.dart

Lines 49 to 50 in d99cc3c

	/// [contentType] currently defaults to `application/octet-stream`, but in the
	/// future may be inferred from [filename].