Only cache the mimetype if the file exists

Otherwise files access control will cache "application/octet-stream"
for all the target node on:
rename, move, copy and all other methods which create a new item

To check this:
1. Add an automated tagging rule which tags on mimetype NOT "httpd/unix-directory"
2. Add an access control rule which checks for any mimetype
3. Create a folder and rename it, the folder should not be tagged, but it is

Signed-off-by: Joas Schilling <coding@schilljs.com>