Discard empty upload_name before persisting file

changelog.d/7905.bugfix

@@ -0,0 +1 @@
+Fix a longstanding bug when storing a media file with an empty `upload_name`.

synapse/rest/media/v1/media_repository.py

@@ -139,23 +139,24 @@ def mark_recently_accessed(self, server_name, media_id):
     async def create_content(
         self,
         media_type: str,
-        upload_name: str,
+        upload_name: Optional[str],
         content: IO,
         content_length: int,
         auth_user: str,
     ) -> str:
         """Store uploaded content for a local user and return the mxc URL
 
         Args:
-            media_type: The content type of the file
-            upload_name: The name of the file
+            media_type: The content type of the file.
+            upload_name: The name of the file, if provided.
             content: A file like object that is the content to store
             content_length: The length of the content
             auth_user: The user_id of the uploader
 
         Returns:
             The mxc url of the stored content
         """
+
         media_id = random_string(24)
 
         file_info = FileInfo(server_name=None, file_id=media_id)

synapse/rest/media/v1/upload_resource.py

@@ -63,6 +63,10 @@ async def _async_render_POST(self, request):
                     msg="Invalid UTF-8 filename parameter: %r" % (upload_name), code=400
                 )
 
+        # If the name is falsey (e.g. an empty byte string) ensure it is None.
+        else:
+            upload_name = None
+
         headers = request.requestHeaders
 
         if headers.hasHeader(b"Content-Type"):