Use file content_type when file is ActionDispatch::Http::UploadedFile

@steved555 @grosser @mcodella 
https://zendesk.atlassian.net/browse/HC-2715

When uploading attachments using ActionDispatch::Http::UploadedFile, the temp_file path does not resolve to any mime type - i.e. `/var/folders/6l/2c1qltgj1p3526rgkkb2vpkr0000gp/T/RackMultipart20140310-86212-16fkt8o` - but the ActionDispatch::Http::UploadedFile has the correct content_type.

This should take care of that case. Let me know if this shouldn't be taken care of as a general solution in this gem.