diff --git a/synapse/rest/media/v1/media_repository.py b/synapse/rest/media/v1/media_repository.py
index 45bc534200..97c82c150e 100644
--- a/synapse/rest/media/v1/media_repository.py
+++ b/synapse/rest/media/v1/media_repository.py
@@ -146,12 +146,10 @@ class MediaRepository(object):
             media_length=content_length,
             user_id=auth_user,
         )
-        media_info = {
-            "media_type": media_type,
-            "media_length": content_length,
-        }
 
-        yield self._generate_thumbnails(None, media_id, media_info)
+        yield self._generate_thumbnails(
+            None, media_id, media_id, media_type,
+        )
 
         defer.returnValue("mxc://%s/%s" % (self.server_name, media_id))
 
@@ -227,6 +225,34 @@ class MediaRepository(object):
         else:
             respond_404(request)
 
+    @defer.inlineCallbacks
+    def get_remote_media_info(self, server_name, media_id):
+        """Gets the media info associated with the remote file, downloading
+        if necessary.
+
+        Args:
+            server_name (str): Remote server_name where the media originated.
+            media_id (str): The media ID of the content (as defined by the
+                remote server).
+
+        Returns:
+            Deferred[dict]: The media_info of the file
+        """
+        # We linearize here to ensure that we don't try and download remote
+        # media multiple times concurrently
+        key = (server_name, media_id)
+        with (yield self.remote_media_linearizer.queue(key)):
+            responder, media_info = yield self._get_remote_media_impl(
+                server_name, media_id,
+            )
+
+        # Ensure we actually use the responder so that it releases resources
+        if responder:
+            with responder:
+                pass
+
+        defer.returnValue(media_info)
+
     @defer.inlineCallbacks
     def _get_remote_media_impl(self, server_name, media_id):
         """Looks for media in local cache, if not there then attempt to
@@ -257,6 +283,7 @@ class MediaRepository(object):
         # If we have an entry in the DB, try and look for it
         if media_info:
             if media_info["quarantined_by"]:
+                logger.info("Media is quarantined")
                 raise NotFoundError()
 
             responder = yield self.media_storage.fetch_media(file_info)
@@ -384,7 +411,7 @@ class MediaRepository(object):
         }
 
         yield self._generate_thumbnails(
-            server_name, media_id, media_info
+            server_name, media_id, file_id, media_type,
         )
 
         defer.returnValue(media_info)
@@ -496,21 +523,22 @@ 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_type,
+                             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_type (str): The content type of the file
+            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"]
-        file_id = media_info.get("filesystem_id")
         requirements = self._get_thumbnail_requirements(media_type)
         if not requirements:
             return
@@ -568,7 +596,7 @@ class MediaRepository(object):
             try:
                 file_info = FileInfo(
                     server_name=server_name,
-                    file_id=media_id,
+                    file_id=file_id,
                     thumbnail=True,
                     thumbnail_width=t_width,
                     thumbnail_height=t_height,
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index f3dbbb3fec..981f01e417 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -185,8 +185,10 @@ 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["media_type"],
+                url_cache=True,
             )
 
             og = {
@@ -231,8 +233,10 @@ 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["media_type"],
+                        url_cache=True,
                     )
                     if dims:
                         og["og:image:width"] = dims['width']
diff --git a/synapse/rest/media/v1/thumbnail_resource.py b/synapse/rest/media/v1/thumbnail_resource.py
index 835540c3dd..8c9653843b 100644
--- a/synapse/rest/media/v1/thumbnail_resource.py
+++ b/synapse/rest/media/v1/thumbnail_resource.py
@@ -84,7 +84,11 @@ class ThumbnailResource(Resource):
                                  method, m_type):
         media_info = yield self.store.get_local_media(media_id)
 
-        if not media_info or media_info["quarantined_by"]:
+        if not media_info:
+            respond_404(request)
+            return
+        if media_info["quarantined_by"]:
+            logger.info("Media is quarantined")
             respond_404(request)
             return
 
@@ -111,6 +115,7 @@ class ThumbnailResource(Resource):
             responder = yield self.media_storage.fetch_media(file_info)
             yield respond_with_responder(request, responder, t_type, t_length)
         else:
+            logger.info("Couldn't find any generated thumbnails")
             respond_404(request)
 
     @defer.inlineCallbacks
@@ -119,7 +124,11 @@ class ThumbnailResource(Resource):
                                             desired_type):
         media_info = yield self.store.get_local_media(media_id)
 
-        if not media_info or media_info["quarantined_by"]:
+        if not media_info:
+            respond_404(request)
+            return
+        if media_info["quarantined_by"]:
+            logger.info("Media is quarantined")
             respond_404(request)
             return
 
@@ -149,7 +158,7 @@ class ThumbnailResource(Resource):
                     yield respond_with_responder(request, responder, t_type, t_length)
                     return
 
-        logger.debug("We don't have a local thumbnail of that size. Generating")
+        logger.debug("We don't have a thumbnail of that size. Generating")
 
         # Okay, so we generate one.
         file_path = yield self.media_repo.generate_local_exact_thumbnail(
@@ -159,13 +168,14 @@ class ThumbnailResource(Resource):
         if file_path:
             yield respond_with_file(request, desired_type, file_path)
         else:
+            logger.warn("Failed to generate thumbnail")
             respond_404(request)
 
     @defer.inlineCallbacks
     def _select_or_generate_remote_thumbnail(self, request, server_name, media_id,
                                              desired_width, desired_height,
                                              desired_method, desired_type):
-        media_info = yield self.media_repo.get_remote_media(server_name, media_id)
+        media_info = yield self.media_repo.get_remote_media_info(server_name, media_id)
 
         thumbnail_infos = yield self.store.get_remote_media_thumbnails(
             server_name, media_id,
@@ -181,7 +191,7 @@ class ThumbnailResource(Resource):
 
             if t_w and t_h and t_method and t_type:
                 file_info = FileInfo(
-                    server_name=None, file_id=media_id,
+                    server_name=server_name, file_id=media_info["filesystem_id"],
                     thumbnail=True,
                     thumbnail_width=info["thumbnail_width"],
                     thumbnail_height=info["thumbnail_height"],
@@ -197,7 +207,7 @@ class ThumbnailResource(Resource):
                     yield respond_with_responder(request, responder, t_type, t_length)
                     return
 
-        logger.debug("We don't have a local thumbnail of that size. Generating")
+        logger.debug("We don't have a thumbnail of that size. Generating")
 
         # Okay, so we generate one.
         file_path = yield self.media_repo.generate_remote_exact_thumbnail(
@@ -208,6 +218,7 @@ class ThumbnailResource(Resource):
         if file_path:
             yield respond_with_file(request, desired_type, file_path)
         else:
+            logger.warn("Failed to generate thumbnail")
             respond_404(request)
 
     @defer.inlineCallbacks
@@ -216,7 +227,7 @@ class ThumbnailResource(Resource):
         # TODO: Don't download the whole remote file
         # We should proxy the thumbnail from the remote server instead of
         # downloading the remote file and generating our own thumbnails.
-        yield self.media_repo.get_remote_media(server_name, media_id)
+        media_info = yield self.media_repo.get_remote_media_info(server_name, media_id)
 
         thumbnail_infos = yield self.store.get_remote_media_thumbnails(
             server_name, media_id,
@@ -227,7 +238,7 @@ class ThumbnailResource(Resource):
                 width, height, method, m_type, thumbnail_infos
             )
             file_info = FileInfo(
-                server_name=None, file_id=media_id,
+                server_name=server_name, file_id=media_info["filesystem_id"],
                 thumbnail=True,
                 thumbnail_width=thumbnail_info["thumbnail_width"],
                 thumbnail_height=thumbnail_info["thumbnail_height"],
@@ -241,6 +252,7 @@ class ThumbnailResource(Resource):
             responder = yield self.media_storage.fetch_media(file_info)
             yield respond_with_responder(request, responder, t_type, t_length)
         else:
+            logger.info("Failed to find any generated thumbnails")
             respond_404(request)
 
     def _select_thumbnail(self, desired_width, desired_height, desired_method,