Well, Canvas Help came through and I'll post their answer here, in case it helps someone else. The students had indeed identified a "comment" field on their assignments, but that one, in that location, does NOT allow them to attach files. Instead, they needed to navigate to the assignment's SUBMISSION DETAILS page and scroll to the bottom. That's where they can find a comment field that does in fact sport either a nifty paper clip icon or the words "Attach File." Boom! They can attach files like pros now.
As always, I'd given my students the link How do I view assignment comments from my instructor? -- but they hadn't read through it and assumed they were in the right spot. Understandable. Turns out I needed help interpreting their screenshots, too. Thanks, Johnny! LMGTFY, for sure. But I did need the help.