diff --git a/synapse/media/v1/base_resource.py b/synapse/media/v1/base_resource.py
index 2f5440ab6..38e01970c 100644
--- a/synapse/media/v1/base_resource.py
+++ b/synapse/media/v1/base_resource.py
@@ -201,7 +201,8 @@ class BaseMediaResource(Resource):
         defer.returnValue(media_info)
 
     @defer.inlineCallbacks
-    def _respond_with_file(self, request, media_type, file_path):
+    def _respond_with_file(self, request, media_type, file_path,
+                           file_size=None):
         logger.debug("Responding with %r", file_path)
 
         if os.path.isfile(file_path):
@@ -215,13 +216,20 @@ class BaseMediaResource(Resource):
             request.setHeader(
                 b"Cache-Control", b"public,max-age=86400,s-maxage=86400"
             )
+            if file_size is None:
+                stat = os.stat(file_path)
+                file_size = stat.st_size
+
+            request.setHeader(
+                b"Content-Length", b"%d" % (file_size,)
+            )
 
             with open(file_path, "rb") as f:
                 yield FileSender().beginFileTransfer(f, request)
 
             request.finish()
         else:
-            self._respond_404()
+            self._respond_404(request)
 
     def _get_thumbnail_requirements(self, media_type):
         if media_type == "image/jpeg":
diff --git a/synapse/media/v1/download_resource.py b/synapse/media/v1/download_resource.py
index f3a6804e0..8b5072ebb 100644
--- a/synapse/media/v1/download_resource.py
+++ b/synapse/media/v1/download_resource.py
@@ -46,23 +46,29 @@ class DownloadResource(BaseMediaResource):
     def _respond_local_file(self, request, media_id):
         media_info = yield self.store.get_local_media(media_id)
         if not media_info:
-            self._respond_404()
+            self._respond_404(request)
             return
 
         media_type = media_info["media_type"]
+        media_length = media_info["media_length"]
         file_path = self.filepaths.local_media_filepath(media_id)
 
-        yield self._respond_with_file(request, media_type, file_path)
+        yield self._respond_with_file(
+            request, media_type, file_path, media_length
+        )
 
     @defer.inlineCallbacks
     def _respond_remote_file(self, request, server_name, media_id):
         media_info = yield self._get_remote_media(server_name, media_id)
 
         media_type = media_info["media_type"]
+        media_length = media_info["media_length"]
         filesystem_id = media_info["filesystem_id"]
 
         file_path = self.filepaths.remote_media_filepath(
             server_name, filesystem_id
         )
 
-        yield self._respond_with_file(request, media_type, file_path)
+        yield self._respond_with_file(
+            request, media_type, file_path, media_length
+        )
diff --git a/synapse/media/v1/thumbnail_resource.py b/synapse/media/v1/thumbnail_resource.py
index 5ddcf54b9..666764203 100644
--- a/synapse/media/v1/thumbnail_resource.py
+++ b/synapse/media/v1/thumbnail_resource.py
@@ -100,11 +100,12 @@ class ThumbnailResource(BaseMediaResource):
             t_type = thumbnail_info["thumbnail_type"]
             t_method = thumbnail_info["thumbnail_method"]
             file_id = thumbnail_info["filesystem_id"]
+            t_length = thumbnail_info["thumbnail_length"]
 
             file_path = self.filepaths.remote_media_thumbnail(
                 server_name, file_id, t_width, t_height, t_type, t_method,
             )
-            yield self._respond_with_file(request, t_type, file_path)
+            yield self._respond_with_file(request, t_type, file_path, t_length)
         else:
             yield self._respond_default_thumbnail(
                 request, media_info, width, height, method, m_type,
@@ -139,11 +140,12 @@ class ThumbnailResource(BaseMediaResource):
         t_height = thumbnail_info["thumbnail_height"]
         t_type = thumbnail_info["thumbnail_type"]
         t_method = thumbnail_info["thumbnail_method"]
+        t_length = thumbnail_info["thumbnail_length"]
 
         file_path = self.filepaths.default_thumbnail(
             top_level_type, sub_type, t_width, t_height, t_type, t_method,
         )
-        yield self.respond_with_file(request, t_type, file_path)
+        yield self.respond_with_file(request, t_type, file_path, t_length)
 
     def _select_thumbnail(self, desired_width, desired_height, desired_method,
                           desired_type, thumbnail_infos):