The first step toward "auto-downloading" is ensuring Thunderbird actually retrieves the full content of your emails, including attachments, rather than just the headers. This is critical for IMAP users who want their files available offline immediately.
To truly automate the process—where a file arrives and is instantly copied to a folder—you'll likely need an extension that adds new "Actions" to Thunderbird's built-in filtering system. thunderbird auto download attachments
In the panel, scroll down to the Files & Attachments section. thunderbird auto download attachments
: This popular add-on adds a "Save Attachments" action to your message filters. thunderbird auto download attachments