adding .htm extension to post types in CPT UI

I would check out a tutorial like http://carlofontanos.com/add-html-extension-to-permalinks/ for that topic. It’s not something that should be done via the rewrite slug field, from everything I’ve always seen. It’s more a permalinks setting/customization.

Not that I’ve ever been aware of.

Just for the sake of really quick example, say the “folder” in domain.com/folder/%postname.htmis your post type. Setting the custom rewrite slug to “manilla-folder” would make your url then be domain.com/manilla-folder/%postname.htm It would not be part of what affects the end of the resulting url and the appended .htm/.html

I can’t say with any certainty that it would or not, but it’s what I’d call a “dirty” way to accomplish if it appears to work. It’s trying to put a round peg into a square hole. The link/tutorial I provided above would be a more appropriate way to go about this permalink customization, in my professional opinion.