This is a design time configuration of attachment category, I don't see any reason why all categories(in current hierarchy) shouldn't be listed in there. The list is what is shown in OOTB attachment gadget as well. Do you have any business justification to show limited set of categories?
If you do not select any category, default 'File' category is used at runtime.
You can also use property reference and whatever is the value of property at runtime will be the category used while saving attachment.
Justification is: We do not want users to set other categories other than the one we set because we have a section which we need to show files in the category only. I also remember that we were able to fix the category before...
Anyway, I also submit SR to check whether there is a way to archive what we want to do.