Mock File object during tests to be compatible with FormData#append

Not completely sure why, but probably because of browser version difference, the tests failed for us.
(Tried with Chrome v36.0.1985.125 & Firefox 30.0)
We found that plain JavaScript objects are not accepted by the append method of the FormData "class".
Since we suspect it's impossible to manually create new File objects, we faked it with a new Blob extended with a name property to mimic the File interface.