It's the other way around: when you click the Attach File link above the description, you can submit a comment along with the uploaded file.
No. I mean on the original post, I am allowed to attach one file and only one file (with a description for that one file). And then if I reply to my original post, there is NO option to attach a file. I had to use URL links in the reply.
are the attached screenshots from a high-DPI (display density) device?
I'm not 100% sure what you mean by a high-DPI device. I have a 15.6" screen laptop. By default, it comes with a 1920x1080 resolution. For the last year, I have been using 1366x768 resolution. Yesterday, or the day before, I switched to 1600x900 resolution. Just now, I tested all three resolutions. The results are the same in all of them.
I just want to add that this is a Firefox issue. Google Chrome doesn't have this problem. I haven't tested Internet Explorer or Microsoft Edge because the only browsers I use are Firefox and Google Chrome. And I already mentioned this, but I tested this out in both Firefox 56.0.2 and Firefox 67. And it was the same in both cases.
I don't think this is a matter of high-DPI or low-DPI. I think this is a matter of Firefox simply not displaying input spinners the same way other browsers do, which causes this "you can't see it because it is too small" problem.