Fixes test suite failure under win32 (issue #275)

`FormParserTestCase.test_large_file` failed on win32 because stdlib's `TemporaryFile` returns a `_TemporaryFileWrapper` and not a `file` as it does under posix systems. 
With this fix, the test checks that the stream has a `fileno` attribute instead of checking that it's a file with `isinstance(..., file)`.