Comment 28 for bug 303140

Do not know if this is related to your problem but while searching for duplicates I found this report in Bugzilla#453490:

The "Detach all" and "Delete All" menu items for email attachments are always disabled (gray), both in the context menu for the list of attachments in the message header page, and in the Message->Attachments menu (from the menu bar). This makes it difficult to delete large numbers of attachments on emails.

Today, I went back to the email that had this problem (could not detach/delete attachments) and I happened to click in the header pane to the left of the attachment list box. When I did so, an interesting thing happened. The icon for a signed email appeared there, where it had not been before. I do not know why that icon did not appear until I clicked there, but that's what happened.

So, then I searched for bugs and found:
You can't Detach (which allows saving the attachments) or Delete the attachments from smime mails (signed or encrypted), at least on trunk it's disabled.