fix: Use f-string to show filename in error message correctly (#7589)

Co-authored-by: Philipp Rudiger <[email protected]>

panel/command/convert.py

@@ -114,7 +114,7 @@ def invoke(self, args: argparse.Namespace) -> None:
         for f in args.files:
             p = pathlib.Path(f).absolute()
             if not p.is_file():
-                raise ValueError('File {f!r} not found.')
+                raise ValueError(f'File {f!r} not found.')
             elif p not in excluded:
                 included.append(p)