Managing your email attachments in Windows Thunderbird
Step 1. Open the message with the attachment, right-click on the attachment icon and choose 'Detach...':
Step 2. You will be prompted for the download/save location. If you need to access the attachment from multiple machines, ensure that this location is a network drive.
Step 3. Confirm that you wish to detach the attachment from the message:
The mail message will still exist in your mailbox, but the attachment will be a local copy, as you will see when you try to open it:
Note that the file location is not imap://imap.hermes.cam.ac.uk:993/, but the location you selected in Step 2.: