fix for #47 - upload button doesnt work on rails;

Hi,
here is cleaned up and tested version of fix for issue #47 .

Yes, it's just one line and it's caused by fact that Rails uses `Hash` instead of 'Rack::Multipart::UploadedFile'. 

And i closed my previous PR #48 -  as it's not relevant anymore. 

This should be a CONSTANT.