Is it possible to delete an attachment without MySql ?
All database backends have a ::DELETE() call, and so this is independent from MySQL. If you found out the internal database name of the attachment, you could use the WikiCommander or tools/ewikictl for example.
Ok, but for the average user of the wiki there is no way to delete an old or wrong attachement ?
If that is so, isn't that a lack of functionality !
Sure, but while text pages in a Wiki are openly editable, it would be dangerous to allow users to purge stuff and grant full access to database administration.
The attachment feature is one of the more exotic (=non-core) functions and its current implementation just an ad-hoc extension, not well integrated with anything. We do not have a standardized means to connect database entries to each other, and so the association (and even the existence) of attachments is rather loose.
Instead of deletion it would be interesting to have the ability to disconnect attachments and put them into some sort of Trashcan. Just has never been implemented (and I have no time for it).