Forms: added XHR2 FormData support for file upload

[XHR2 using FormData](https://developer.mozilla.org/en-US/docs/Web/Guide/Using_FormData_Objects#Sending_files_using_a_FormData_object)
Forms: use FormData only for POST method
Merge pull request #96 from Ciki/patch-1

Added XHR2 FormData support for file upload.