“Category” is a taxonomy. What you’re looking to do is limit the number of “terms”
Depending on your goal (I think you want to show comingled attachments and posts in one giant list/interface?) – what you could do is create a new taxonomy, call my_attachment_terms and then add terms to that taxonomy and label them appropriately, **utilize the wp_insert_term ** function – specifically marking it as an alias of an existing term.
https://codex.wordpress.org/Function_Reference/wp_insert_term
‘alias_of’
(string) (optional) There is no default, but if added, expected is the slug that the term will be an alias of.
Default: None
I’m not sure what the implications of using this on the OOTB category taxonomy is, so I would recommend working with custom taxonomies first, if you do not have a dev environment that you can wipe and reset at a whim
good luck!