Relationship between files and categories is done through parent_id relationship. In case of attachments, this field is used to define which object attachments belong to so it cannot be used for connection with categories.
Apart from that technical limitation, system is defined so attachments are organized by parent objects and it was never intended for them to be sorted in categories.
Thanks for your answer, actually we use file attachment on ticket like a shorcut to post a file to a project. Perhaps a another simple solution is to let user specify a file category when the 'quick add' fonction is used. What do you think about this ?
Attachments are just that - attachments. Reason why they are listed on Files list is for convenience. We do understand that posting attachments is sometimes easier that creating a file, but if you need more firepower (like categorization, summary, comments, email notifications, version tracking etc), use files. That's what they are designed for.
I think just a good thing is to let user categorize file when we use the "Quick Add" button. As we have to choose the project, it can be simple to purpose a dropdown menu with the category available on this project.
Oh, sorry - I missed that. Category picker is more likely to be added than categories for attachments. We'll consider this feature for future releases.
+1 here. The ability to pick a category when attaching files is something we'd really use a lot. At a minimum the ability to change the category later on if need be.