re-order thumbnail keys

event-schemas/schema/m.room.message#m.file

@@ -20,15 +20,15 @@ properties:
           size:
             description: The size of the file in bytes.
             type: integer
+          thumbnail_url:
+            description: The URL to the thumbnail of the file.
+            type: string
           thumbnail_info:
             allOf:
               - $ref: core-event-schema/msgtype_infos/thumbnail_info.yaml
             description: Metadata about the image referred to in ``thumbnail_url``.
             title: ThumbnailInfo
             type: object
-          thumbnail_url:
-            description: The URL to the thumbnail of the file.
-            type: string
         title: FileInfo
         type: object
       msgtype: