The Permanently Detach feature is currently not recommend for use with Gmail accounts

[tmccoid-tech]

A popular purpose of the "Permanently Detach" feature of Extract 'Em! is to free disk space, particularly with respect to IMAP accounts where messages are stored on an email provider's server which enforce quotas.

Unfortunately, due to the interaction between Thunderbird and Gmail's labeling system (in particular the All Mail label), deleting an attachment from a message in the Inbox does not result in the space being reclaimed.

When a message is received by Gmail, it appears not only in the Inbox but also under All Mail, and although the two references pertain to the same underlying message, Thunderbird appears to interpret the references as two distinct messages.

Therefore, when Thunderbird deletes an attachment -- which requires creating a duplicate and modifying it to eliminate the attachment data, and then deleting the original -- the result is that the correctly modified message displays in the Inbox, but All Mail retains the original message AND also displays the modified message.

The result is that no space is reclaimed, confusing and frustrating users.

A bug report has been filed with Mozilla regarding this problem:
Special Gmail labels cause message duplication when deleting attachments

Thanks to the two users who reported this issue and provided feedback:
Emails with attachments in Gmail remain on server after deletion

There is no ETA for a resolution as it is beyond the scope of the add-on and must be addressed at the application (Thunderbird) level.