Improved the uploading of file in regard to temporary filename.

Avoid to overwrite the same file for concurrent requests uploading the same file name.

Fixing issue https://github.com/EiffelWebFramework/EWF/issues/132
I think in this case, it should be guaranteed ...

it is unlikely the execution raises an exception and get rescued .. on the "Result := f"
so if Result is assigned ... this is because it is `is_open_write', so the postcondition should be ok, even if there is a rescue.