Use the right path for url_preview thumbnails

This was introduced by #2627: we were overwriting the original media for url
previews with the thumbnails :/

(fixes element-hq/element-web#6012, hopefully)

synapse/rest/media/v1/media_storage.py

@@ -164,6 +164,14 @@ def _file_info_to_path(self, file_info):
             str
         """
         if file_info.url_cache:
+            if file_info.thumbnail:
+                return self.filepaths.url_cache_thumbnail_rel(
+                    media_id=file_info.file_id,
+                    width=file_info.thumbnail_width,
+                    height=file_info.thumbnail_height,
+                    content_type=file_info.thumbnail_type,
+                    method=file_info.thumbnail_method,
+                )
             return self.filepaths.url_cache_filepath_rel(file_info.file_id)
 
         if file_info.server_name: