diff --git a/changelog.d/5915.bugfix b/changelog.d/5915.bugfix
new file mode 100644
index 000000000..bf5b99fed
--- /dev/null
+++ b/changelog.d/5915.bugfix
@@ -0,0 +1 @@
+Fix 404 for thumbnail download when `dynamic_thumbnails` is `false` and the thumbnail was dynamically generated. Fix reported by rkfg.
diff --git a/synapse/rest/media/v1/media_repository.py b/synapse/rest/media/v1/media_repository.py
index d4ea09260..b972e152a 100644
--- a/synapse/rest/media/v1/media_repository.py
+++ b/synapse/rest/media/v1/media_repository.py
@@ -526,7 +526,7 @@ 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,