Update secure_downloads.rst

Enable mod_xsendfile in apache configuration
Merge pull request #431 from amorce/patch-1

Update secure_downloads.rst
Fix mismatched double quotes in filer admin templates
Merge pull request #434 from aptivate/fix_mismatched_quotes

Fix mismatched double quotes in filer admin templates
Clarify static files
Merge pull request #437 from nephila/feature/staticdoc

Clarify static files
added missing unlocalize filters
Merge pull request #444 from jrief/unlocalize-fixes

added missing unlocalize filters