[web2py] Why the temporary file is larger than the original one?

[Charles Tang]

I am uploading a video from using web2py SQLFORM and want to process it 
while it is uploading. I find that even the video is completely uploaded 
the temporary file of the video in the /tmp directory is larger than the 
original video.Why?Does the python tempfile module add some bytes to 
it?Many thanks!