Currently, Mattermost provides the ability to edit a message after you have posted it.

However, there is no way to modify the file attachments via this editing process.

Recent use case: I uploaded the wrong file when posting a message. Ideally I would have been able to edit the post to correct the mistake, i.e. to remove the incorrect file and attach the one I meant to attach. However, I could find no way of doing this.

Luckily, in my case it was simply an old version of the document, and therefore not too big a problem. However, I am sure there will be situations where someone incorrectly posts a document containing sensitive information, and in this situation the ability to delete the file is paramount, for security reasons.

Another option would be to just add the ability to delete a file, as you can always repost the correct file as a new message. This is a sub-optimal solution but it would be preferable to a long delay in order to implement a full solution.

Functionally, I would expect this to work using the existing rules about who can edit a message (and when). The message editing interface would simply be updated to show the attached files (which can be removed) and to allow new files to be added, in an identical manner as when creating a new message.

(In fact, this ticket could be treated as 'make the interface for editing an existing message the same as the interface for adding a new message, if that is an easier way of handling it.)