Such request can happen on Internet Explorer. When we redirect
after multipart form submission, the request type is changed
to GET, but Content-Type is preserved as multipart. GET request
cannot have multipart body and that caused Rails to fail.

It's similar fix to Rack's one:
https://github.com/chneukirchen/rack/blob/8025a4ae9477d1e6231344c2b7d795aa9b3717b6/lib/rack/request.rb#L224