2 files changed, 4 insertions, 1 deletions
diff --git a/synapse/media/_base.py b/synapse/media/_base.py
index d103b43449..13345acf75 100644
--- a/synapse/media/_base.py
+++ b/synapse/media/_base.py
@@ -332,7 +332,7 @@ class ThumbnailInfo:
     # Content type of thumbnail, e.g. image/png
     type: str
     # The size of the media file, in bytes.
-    length: Optional[int] = None
+    length: int
 
 
 @attr.s(slots=True, frozen=True, auto_attribs=True)
diff --git a/synapse/media/media_repository.py b/synapse/media/media_repository.py
index d11c2ff4ee..7fd46901f7 100644
--- a/synapse/media/media_repository.py
+++ b/synapse/media/media_repository.py
@@ -624,6 +624,7 @@ class MediaRepository:
                         height=t_height,
                         method=t_method,
                         type=t_type,
+                        length=t_byte_source.tell(),
                     ),
                 )
 
@@ -694,6 +695,7 @@ class MediaRepository:
                         height=t_height,
                         method=t_method,
                         type=t_type,
+                        length=t_byte_source.tell(),
                     ),
                 )
 
@@ -839,6 +841,7 @@ class MediaRepository:
                         height=t_height,
                         method=t_method,
                         type=t_type,
+                        length=t_byte_source.tell(),
                     ),
                 )