Currently, there seems to be no way to recover files that have been directly attached to a task. However, since accidental deletions can occur, it would be very helpful if recovery became possible.
Similar ideas can be found here.
削除した添付ファイルを復元したい - Japanese Community / 製品フィードバック - Asana Community
Similar questions can be found here.
Recover attachments - Asana Community / Ask the Community - Asana Community
If a similar idea already exists, please integrate it there.