I have a feeling Windows 7 works differently than Windows 10, but anyway, I fixed it. I must confess I got some tips by using a search engine. The “trick” was setting Thunderbird so that it would work with Internet Explorer v. 11.
In IE under Internet Options, there’s a tab called programs. Even though I had already set Thunderbird as the default email program using the Windows Default Programs interface, IE didn’t work until I set it in this tab. And when it worked in IE, it worked in Sumatra, too.
The crazy thing is that now when I right-click a file in Windows Explorer and then SendTo, There’s no option to send the file via email, but that’s OK.
At least Sumatra works correctly now. I’m going to reboot my computer now, though, just to make sure.