fix: respect access_type in shared-chat file authorization branch (#24755)

has_access_to_file granted access whenever the file was attached to a
shared chat the user could read, ignoring the requested access_type. A
read-only shared-chat recipient therefore satisfied write and delete
checks and could delete or mutate the chat owner's attached file. Gate
the shared-chat branch on read access, matching the channels branch
directly above it.

Co-authored-by: oxsignal <oxsignal@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/open_webui/utils/access_control/files.py

@@ -66,7 +66,7 @@ async def has_access_to_file(
 
     # Check if the file is associated with any chats the user has access to
     shared_chat_ids = await Chats.get_shared_chat_ids_by_file_id(file_id, db=db)
-    if shared_chat_ids:
+    if access_type == 'read' and shared_chat_ids:
         accessible_ids = await AccessGrants.get_accessible_resource_ids(
             user_id=user.id,
             resource_type='shared_chat',