Open (cat) on missing file causes str vs bytes issue

[jonasbardino]

It appears cat on missing file can cause str vs byte issues like:

2026-03-06 11:54:34,649 ERROR file formatting failed: can't concat str to bytes
2026-03-06 11:54:34,649 ERROR original 'cat' response was: [{'object_type': 'start', 'headers': '[ ... ]'}, {'object_type': 'title', 'text': 'cat', 'meta': '', 'style': '{ ... }', 'script': '{ ... }', 'backend': 'cat', 'base_menu': '[ ... ]', 'user_settings': '{ ... }', 'user_profile': '{ ... }', 'user_menu': '[ ... ]'}, {'object_type': 'header', 'text': 'cat'}, {'object_type': 'file_not_found', 'name': 'path/to/missing/file.txt'}, {'object_type': 'timing_info', 'text': 'done in 1.250s'}, {'object_type': 'wsgi', 'environ': {...}}]

The problem is in file_format - it sets a variable to be b'' with a comment above noting about how everything is binary and then it promptly attempts to append strings. I don't think a non-existent file is the only case that will trigger such a problem either.