What type of file attachment are you sending? eg: excel ? what is the extension ? Are you using the Thunderbird Write window when you select to attach the file ? OR the excel menu FILE: SEND TO: Mail Recipient as attachment ? If the latter via excel, then do you get the same issue if you create and attach the file via Thunderbird Write window?

In your 'Sent' folder, does the email you sent show the correct attachment extension ?

In every case it was docx or doc file sent via the compose window. I had written a message and added the attachment in the simplest way possible.