UI: Blend File Icons Thumbnail View

Changes icon used to indicate blend file when overlaid over larger
document icon when in thumbnail view. Only seen when file does not
have a preview.

Followup to {rB611e4ffaab43}

For more details and examples see D13342

Differential Revision: https://developer.blender.org/D13342

Reviewed by Julian Eisel

source/blender/editors/space_file/filelist.c

@@ -1351,7 +1351,7 @@ static int filelist_geticon_ex(const FileDirEntry *file,
   }
 
   if (typeflag & FILE_TYPE_BLENDER) {
-    return ICON_FILE_BLEND;
+    return (is_main || file->preview_icon_id) ? ICON_FILE_BLEND : ICON_BLENDER;
   }
   if (typeflag & FILE_TYPE_BLENDER_BACKUP) {
     return ICON_FILE_BACKUP;