Trying to make TRANSFORMABLE_TYPES generic

Trying to get ActionDispatch::Http::UploadedFile objects to work with HTTMultiParty.

This is related to this pull request: https://github.com/jwagener/httmultiparty/pull/27

I don't think this is ready to merge right this moment, but I hope to get some feedback on how this could be made to work and be accepted.

Thanks!