[Proposal] Sign post but skip files

This commit signs post request which have files, but doesn't use them for signing.
I don't know what is in the spec about this, or what is common to do, but the Flickr API for uploading files (https://www.flickr.com/services/api/upload.api.html) uses OAuth 1 and says: 
> Note that the 'photo' parameter should not be included in the signature. All other POST parameters should be included when generating the signature.

I'm not sure if this is the correct way to do it, or that there is another way to accomplish this?