Fall back to fast content type if regular is unavilable in file attributes

This fixes Thunar misrecognizing m3u files on an sftp mount as application/octet-stream on my system.

(in case its relevant: Arch Linux with glib 2.64.5-1 and shared-mime-info 2.0+1+g6bf9e4f-2)

Demonstration of (un)availability of content type attributes:

$ gio info /run/user/1000/gvfs/sftp:host=nas.lan/tmp/test.m3u | grep content-
  standard::fast-content-type: audio/x-mpegurl
$ gio info /tmp/test.m3u | grep content-
  standard::content-type: audio/x-mpegurl
  standard::fast-content-type: audio/x-mpegurl