REG: restore format_type attr
- [x] closes #30962

cc @TomAugspurger 
Thanks. Two questions:

1. Thoughts on the need for a simple test? I worry a bit about a comment being overlooked in a future refactor clearing stuff out / looking at "dead" code.
2. Do we want this API going forward? Clearly, we don't have a need for it. Is there an alternative we should deprecate in favor of?  > 1. Thoughts on the need for a simple test?

code sample in issue as test in pandas\tests\test_downstream.py wrong button! @TomAugspurger suggestions for a snippet to test? Nothing fancy. Just an asset that hdf.get_storer(key).format_type is the expected value for that format Is the purpose of this field documented anywhere? If not can we add a comment? Pushed a test. > Is the purpose of this field documented anywhere? If not can we add a comment?

IMO we can keep PR to just fixing the regression. Thanks @jbrockmendel.