nemo-file.c: Allow files with backend-provided previews to be

thumbnailed, even if their filesystem forbids it.

Nemo's thumbnailer now returns the provided preview icon if one
exists. Actual thumbnail visibility is still controlled by the
user like normal.

This will not guarantee thumbnails, if the device doesn't have
any, but the couple Android devices I tried did have them.

ref:
#3055
#3458
#3291
https://gitlab.gnome.org/GNOME/nautilus/-/issues/1921

Author: mtwebster

libnemo-private/nemo-file-private.h

@@ -33,7 +33,7 @@
 #include <eel/eel-string.h>
 
 #define NEMO_FILE_DEFAULT_ATTRIBUTES				\
-	"standard::*,access::*,mountable::*,time::*,unix::*,owner::*,selinux::*,thumbnail::*,id::filesystem,trash::orig-path,trash::deletion-date,metadata::*"
+	"standard::*,access::*,mountable::*,time::*,unix::*,owner::*,selinux::*,thumbnail::*,id::filesystem,trash::orig-path,trash::deletion-date,metadata::*,preview::icon"
 
 /* These are in the typical sort order. Known things come first, then
  * things where we can't know, finally things where we don't yet know.
@@ -180,6 +180,7 @@ struct NemoFileDetails
 	eel_boolean_bit got_custom_display_name       : 1;
 	eel_boolean_bit got_custom_activation_uri     : 1;
 
+    eel_boolean_bit has_preview_icon              : 1;
 	eel_boolean_bit thumbnail_is_up_to_date       : 1;
 	eel_boolean_bit thumbnail_wants_original      : 1;
 	eel_boolean_bit thumbnail_tried_original      : 1;

libnemo-private/nemo-file.c

@@ -2675,6 +2675,11 @@ update_info_internal (NemoFile *file,
 		file->details->trash_orig_path = g_strdup (trash_orig_path);
 	}
 
+    if (g_file_info_has_attribute (info, G_FILE_ATTRIBUTE_PREVIEW_ICON))
+    {
+        file->details->has_preview_icon = TRUE;
+    }
+
 	changed |=
 		nemo_file_update_metadata_from_info (file, info);
 
@@ -4388,7 +4393,7 @@ nemo_file_should_show_thumbnail (NemoFile *file)
     }
 
 	use_preview = nemo_file_get_filesystem_use_preview (file);
-    if (use_preview == G_FILESYSTEM_PREVIEW_TYPE_NEVER) {
+    if (use_preview == G_FILESYSTEM_PREVIEW_TYPE_NEVER && !file->details->has_preview_icon) {
         /* file system says to never thumbnail anything */
 		return FALSE;
 	}