Add `File` as valid param type

[gi]

With a type declaration of File, the input parameter hash of the uploaded file is coerce into a new UploadedFile object which conforms to the Rack::Multipart::UploadedFile interface.

The input parameter hash must be of the shape:

• filename: the original file name of the uploaded file
• head: the header lines of the multipart request
• name: the parameter name
• tempfile: the Tempfile created from the content of the multipart request
• type: the content media/MIME type

Fixes #103