Opening the trashbin causes errors in log for files without preview

* put a file without a generated preview in the trashbin
  (e.g. a *.docx file)
* open the trashbin
* following errors will show up in the nextcloud.log:
	- filesize(): stat failed for ...
	- fopen(...): failed to open stream: No such file or directory at ...
	- fread() expects parameter 1 to be resource, boolean given at ...
	- fclose() expects parameter 1 to be resource, boolean given at ...
	- imagecreatefromstring(): Empty string or invalid image at ...

This is because the preview code tries to load an SVG image, which
is obviously only text.

The fix simply handles this before the loading happens and the web UI
keeps showing the default mimetype icon.