Thus, they appear as extra unreadable files. If you have more than one signature for an account, Mail can automatically alternate between them — choose At Random or In Sequential Order from the pop-up menu. But this does not affect whether an attachment appears as an icon or a thumbnail. When I saw your terminal command suggestion, I hoped that either the code had changed or the Apple OS updates might allow it to work.
1. Use Mail Drop and Similar Cloud Services
The more minimal it is, the better. Remember, less is more. Click the Choose Signature pop-up menu, then choose a signature.