Ektron 9 - steps to trigger issue ... we think:
The "new" menu on the folder now:
eGandalf gave the simple answer via Twitter (previously I had attempted a workaround of putting inheritance back on then resetting the tree): toggle the "Require Smart Forms" checkbox on the "Smart Forms" tab and the Blank HTML option will come back.
This is by design. When you remove the HTML option, note that it checks the "Require Smart Form" box (and, reciprocally, checking the Require Smart Form box removes the HTML option). That means that all other types, including PageBuilder, DMS documents, or HTML are now forbidden in this folder.
As a best practice, you shouldn't be mixing types in a folder anyway. A lot of people do, but that doesn't make it any more valid. If you've got a folder for Press Releases and you have a Press Release Smart Form, you shouldn't have any other type of content in that folder. The less you mix types, the less filtering you have to do either with the API or post-request via LINQ. As a result, you'll have a more consistently performing site.
In situations like this, where I want the DMS assets and the Smart Form content to a) both be searchable, b) both be managed by the same user/group and c) there's a strong connection between the two (e.g., Whitepaper information Smart Form + PDF asset download), I'll create a child folder and allow DMS assets there. In the Case Study example, the folder structure may look like this:
In contrast, if I don't want the DMS assets to be independently searchable, I'll remove them from the Content folder entirely and place them in a separate, root-level, non-searchable folder called Resources or Downloads.