Support new `upload` field type

GitHub now have this new type:
https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-githubs-form-schema#upload

Using the parser I see this error:
```
Error: Unknown field type: upload
```

It would be good to provide support. Or at least omit it so it doesn't break.

Start at the parser entry point that emits “Unknown field type: upload” and compare its supported field types with GitHub’s upload schema documentation. Define the expected behavior for parsing or omitting upload fields, then verify that existing field parsing still works and that upload no longer breaks processing.

Written by the indexing model from the issue text.