file: Generate thumbnails when the preview icon is available
Currently, thumbnails are not generated for MTP/GPhoto2 locations. The
backends return G_FILESYSTEM_PREVIEW_TYPE_NEVER
in order to prevent
thumbnailers to make them unresponsive. However, the backends provide
preview icons for some file types and thumbnailers should prefer those
icons if available. Let's generate thumbnails for files when the preview
icon is available. This should not affect backend responsiveness much.
Fixes: #1921 (closed)
Edited by Ondrej Holy