I tried both "Delete with normal Thunderbird code" and "Detach with AE's internal routine." Both work great, so to avoid the Thunderbird prompts, I will use AE's routine. It hung once on an image of indeterminate size, which I detached by hand with Thunderbird's code, and then I resumed AE with the rest of the folder. Save some space!
The only issue is that the Size (column view option) that is tallied for the folder does not get updated by either option. So, I only know how many MB I detached by the set of files in my file manager, or by checking in the online webmail client. When one detaches attachments one at a time with the Thundebird code, the folder size updates, but not with AE.
Would be nice if this were fix, as it would help me see how much storage I'm using (because my workplace has a cap on the amount, lame).