I'm grading and providing comments for my students' first assignments. This is my first time using the Grades portion of Canvas. In the column for this particular assignment, there's an image (a sheet of paper) showing the assignment each student uploaded. If I click on that image of the paper, a black dialogue box appears saying "file upload submission," and the image of the paper is replaced with a rectangle with a dash in it. If I click (again) on that rectangle with a dash in it, the image changes to a big checkmark. The checkmark is (I guess) telling me I did something. But what did I do?
What does "file upload submission" mean? What does it mean that the paper image has changed to a checkmark?
I can't figure out what this sequence is doing or why.