If you can get to the Upgrade page under the Admin menu, you will see how much space is being used for each type that counts against the limit, plus the number of Integrations used. Beside the Attachments line, there's a link to View Attachments, if any. That gives you a list of the messages that have one, but you still need to go to each one to delete. (I'd open a new tab with the message.) The default sort is by size, largest first, but you can reverse that or sort by date using the column headers. An owner can always get there, but a moderator would need Billing permissions.
Duane
Duane