Add Content-Type to file fields in HTTP multipart requests

Seems that Unirest (at least the Objective-C version) doesn't sends Content-Type header of files uploaded via HTTP multipart requests. This pull request should fix this.

I'm not entirely sure that `mimeTypeForFileAtPath:` is the right way of guessing the file's MIME Type, I took that code from http://stackoverflow.com/questions/1363813/how-can-you-read-a-files-mime-type-in-objective-c as this doesn't require MobileCoreServices.