Always try and show pre rendered preview

Currently if the following situation happens

Server generates preview
Server has command removed which allows a preview to be shown
Client asks for preview, gets a 404 error when preview exists
(Mime checked before preview)

This happens more often with documents, or video as the commands are not
native PHP, they require a binary on the server.

After the fix the following would happen

Server generates preview
Server has command removed which allows a preview to be shown
Client asks for preview, gets preview which has been generated
(Mime checked after preview)

This would also allow offline generation (for example a docker image
containing the extra binaries), allowing a reduction in attack surface
of the instance serving the preview data.

Signed-off-by: Scott Dutton <scott@exussum.co.uk>