When modifying a message that already has attachments the previously saved attachments can only be deleted if they are removed before new attachments are added/uploaded.
Scenario 1:
1. Edit a message that has saved attachments.
2. Add/upload new attachments.
3. Delete one or more previously saved attachments.
4. Save the message.
The attachments deleted in Step 3 are not removed.
However, if the saved attachments are deleted before new attachments are added/uploaded they are successfully removed from the message.
Scenario 2:
1. Edit a message that has saved attachments.
2. Delete one or more previously saved attachments.
3. Add/upload new attachments.
4. Save the message.
The attachments deleted in Step 2 are removed.
I have confirmed this issue on this forum as well as on my own SMF 2.1 test forums (no mods/custom coding) ...
Fixed in 2.1.3.