Mail Attach Existing Attachment

This module extends the standard email composition window (the one that opens when you click 'Send message' on a record such as a customer, an invoice or a task).

• In that window, next to the usual attachment area, a new list of checkboxes appears showing the files already attached to the record you are writing about. You simply tick the ones you want to send.
• The list is built automatically from the attachments linked to the current record(s) and the current document type, so it always reflects what is really stored on that record.
• The extra section is only shown when the email is being written for an existing record and not in mass mailing mode, so it stays out of the way in other situations.
• When the message is sent, the ticked files are added to the email as regular attachments, so recipients receive them together with the message.

It touches the email composition wizard (mail.compose.message) and the attachment model (ir.attachment), and adds a checkbox field plus a computed list of available attachments. No new menus or reports are created; the change is visible directly inside the standard 'Send message' dialog.