Try not to read uploaded files into memory

[Zverik]

Fixes #634 for most cases. Ideally we could have used os.path.getsize(), but I decided not to introduce any more complexity, just circumvent the reading.

[Zverik]

I actually have no idea what happens when f.seekable() is False. The original code just threw an exception, this code renders the file into an unreadable state.