Saving and detaching attachments
From MozillaZine Knowledge Base
When you receive an attachment—for example, Word documents or photos—Thunderbird keeps it with the e-mail message the attachment came with unless you remove it. To open or view an attachment, delete it, or to save it as a separate file, find the e-mail message it came with and right-click on the attachment at the bottom.
To save attachments for a single e-mail message, you can use any of these methods:
Separating attachments from messages
Beginning with version 1.5, Thunderbird allows you to detach or delete attachments from messages. As opposed to saving an attachment, these actions remove the attachment from the message. (See Bug 2920 for the background). To detach or delete one or more attachments, you can either:
In Thunderbird 1.0x, there is no built-in means to detach or delete an attachment without deleting the entire message itself. You can try the Thunderbird Attachment Tools extension, however.
Most e-mail programs use MIME to attach files to e-mails. MIME converts the file to a format compatible with Internet transmission (i.e. "encodes" it) and attaches it to text of the message, creating one large e-mail. Then the whole e-mail, including the attachment, is sent together.
Thunderbird stores the whole e-mail together, including the attachment, in MIME format in the mailbox files in your Profile folder - Thunderbird. It does not un-encode and store the attachment outside the mailbox file unless you save or detach it as described above. By contrast, the Eudora e-mail program automatically un-encodes and detaches the attachment when you receive the e-mail; it always stores attachments as separate files.
To see an e-mail attachment after MIME processing, send an e-mail with an attachment, then in your Sent folder highlight the e-mail you sent and click "View -> Message Source".
Attachment Extractor extension
The Attachment Extractor extension provides many more features. This includes the ability to filter what attachments should get extracted based on the file name extension. You could use that to extract just the image files in a message or a folder.