Comment 54 for bug 303140

(In reply to Jean from comment #41)
>The size of my mail box is limited and I need to delete some attachments, which I cannot currently.

If doing some one-off housekeeping is urgent for you, a fairly simple workaround would be to use a different e-mail client to delete the offending attachments. You can install it just for this purpose and then uninstall it once you're done. IIRC KMail is able to delete attachments from signed e-mails; probably there are other clients which also have this functionality.