♻️(backend) refactor uploaded file's key format

Remove the user-provided filename while preserving the extension,
and switch to the format `uuid.extension`.

This is more natural than the previous `uuid/.extension` format
and avoids confusion.

Update related tests accordingly.

src/backend/core/tests/files/test_api_files_create.py

@@ -118,7 +118,7 @@ def test_api_files_create_file_authenticated_success():
 
     assert policy_parsed.scheme == "http"
     assert policy_parsed.netloc == "localhost:9000"
-    assert policy_parsed.path == f"/meet-media-storage/files/{file.id!s}/.png"
+    assert policy_parsed.path == f"/meet-media-storage/files/{file.id!s}.png"
 
     query_params = parse_qs(policy_parsed.query)
 

src/backend/core/tests/files/test_api_files_update.py

@@ -39,7 +39,7 @@ def test_api_files_update_anonymous_forbidden():
 
 def test_api_files_update_description_and_title():
     """
-    Test the description and title of an file can be updated.
+    Test the description and title of a file can be updated.
     """
     user = factories.UserFactory()