diff --git a/synapse/rest/media/v1/media_repository.py b/synapse/rest/media/v1/media_repository.py
index d191f9d2f5..35c1dcbc97 100644
--- a/synapse/rest/media/v1/media_repository.py
+++ b/synapse/rest/media/v1/media_repository.py
@@ -151,7 +151,7 @@ class MediaRepository(object):
             "media_length": content_length,
         }
 
-        yield self._generate_thumbnails(None, media_id, media_info)
+        yield self._generate_thumbnails(None, media_id, media_id, media_info)
 
         defer.returnValue("mxc://%s/%s" % (self.server_name, media_id))
 
@@ -413,7 +413,7 @@ class MediaRepository(object):
         }
 
         yield self._generate_thumbnails(
-            server_name, media_id, media_info
+            server_name, media_id, file_id, media_info,
         )
 
         defer.returnValue(media_info)
@@ -525,24 +525,23 @@ class MediaRepository(object):
             defer.returnValue(output_path)
 
     @defer.inlineCallbacks
-    def _generate_thumbnails(self, server_name, media_id, media_info, url_cache=False):
+    def _generate_thumbnails(self, server_name, media_id, file_id, media_info,
+                             url_cache=False):
         """Generate and store thumbnails for an image.
 
         Args:
-            server_name(str|None): The server name if remote media, else None if local
-            media_id(str)
-            media_info(dict)
-            url_cache(bool): If we are thumbnailing images downloaded for the URL cache,
+            server_name (str|None): The server name if remote media, else None if local
+            media_id (str): The media ID of the content. (This is the same as
+                the file_id for local content)
+            file_id (str): Local file ID
+            media_info (dict)
+            url_cache (bool): If we are thumbnailing images downloaded for the URL cache,
                 used exclusively by the url previewer
 
         Returns:
             Deferred[dict]: Dict with "width" and "height" keys of original image
         """
         media_type = media_info["media_type"]
-        if server_name:
-            file_id = media_info["filesystem_id"]
-        else:
-            file_id = media_id
         requirements = self._get_thumbnail_requirements(media_type)
         if not requirements:
             return
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index f3dbbb3fec..5ddf581bd2 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -185,8 +185,9 @@ class PreviewUrlResource(Resource):
         logger.debug("got media_info of '%s'" % media_info)
 
         if _is_media(media_info['media_type']):
+            file_id = media_info['filesystem_id']
             dims = yield self.media_repo._generate_thumbnails(
-                None, media_info['filesystem_id'], media_info, url_cache=True,
+                None, file_id, file_id, media_info, url_cache=True,
             )
 
             og = {
@@ -231,8 +232,9 @@ class PreviewUrlResource(Resource):
 
                 if _is_media(image_info['media_type']):
                     # TODO: make sure we don't choke on white-on-transparent images
+                    file_id = image_info['filesystem_id']
                     dims = yield self.media_repo._generate_thumbnails(
-                        None, image_info['filesystem_id'], image_info, url_cache=True,
+                        None, file_id, file_id, image_info, url_cache=True,
                     )
                     if dims:
                         og["og:image:width"] = dims['width']