Since Slack doesn't support bulk delete, I have searched for alternatives. The search didn't took long, and I stumbled upon this Python script which use Slack's Web API to delete all files that are there for more than 30 days.
After struggling to get the token right, I got a valid token with the scopes files:read and files:write:user.
The script starts, gets the list of files, and sends the request to delete correctly, but the answer from Slack is systematic:
{"ok":false,"error":"cant_delete_file"}
Searching for this error code reveals it is because I don't have the permissions to do erase them.
I can't find any more information, and I'm struggling to know what goes wrong. What is the reason I don't have the permissions?
You need to be authenticated as an admin user to delete a file that belongs to another user.
I would agree that the error message could give more details about the situation.
Edit: this answer is outdated, see this comment.