I am sorry to sound thick. But I have no idea what you are on about. What does seeing if there is an attachment have to do with attachment options? Attachments, when there are any appear at the bottom of the message screen as shown below <img src=https://assets-prod.sumo.prod.webservices.mozgcp.net/media/uploads/images/2025-10-25-19-57-37-1c7aa3.png> This attachment pane appears if there is an attachment. As the file names are set by the sender, Thunderbird needs to know nothing about the file type to receive an attachment. It will remember what application you decide to open the type type in, that is how the external helper application list is grown from the PDF entry that exists because Thunderbird handles them internally. Just as you have no need to know anything about what is inside a package you sign for at your door. Thunderbird does not care anything about attachments. I does not even decode the MIME text that arrives representing the attachment into a file until you decide to do something with it.