Attempting to download large files using send_file with X-Sendfile headers did not work with Python 2.7.2, Apache 2.2.20 on Ubuntu 11.10, only an empty file was downloaded. Setting the "Content-Length" header solved the issue. I can't see why it would hurt to always have it there since response content is discarded when using X-Sendfile anyway.
Added content length to xsendfile
Hmm, of interest:
I guess probably reasonable to do that. If it breaks something we can revert it.
Set the content-length header for sendfile. Fixes #447