Thanks for following up with the Community, Deadwood Al.
 

After performing MorganB's troubleshooting steps, please feel welcome to update me here with your results.
 

To answer your other question, invoices are sent with PDF attachments from QuickBooks. They shouldn't show any other file extensions attached to their invoice emails.
 

Like you speculated in your original post, it's also possible this could be an issue on the recipient's end, instead of something wrong with your template. For example, their email provider could be blocking the PDF attachment for some reason. They'd want to check with their email provider about this possibility.
 

I'll be looking forward to your reply. Have a great Thursday!