To whom it might be useful,
For a standard owncloud installation in /var/www with Apache: Open your /var/www/owncloud/.htaccess in a text editor and append the following lines (Update 2014-10-09 18:35 UTC: Add missing \ before .)
<IfModule mod_headers.c> <FilesMatch "\.(css|js)$"> Header set Cache-Control "max-age=2592000, public" </FilesMatch> </IfModule>
then in a shell make sure that the headers module is enabled in Apache:
sudo a2enmod headers
and restart Apache as prompted by a2enmod.
In Firefox: Press Ctrl-Shift-Q to bring up the Network web developer tool to watch the drama unfold in its entirety. ↩