First, click on the "Add Content Block" button toward the upper left of the screen under your class title.
Then, select the type of content block you want. In this case, I'm going to use a File Block since I specifically want to upload a file. So I click on the "Files" choice.
You will be prompted first to title your file block, then you have the option to click on Block Placement and choose where you want your block to appear on your page. (You can always move it later by dragging and dropping it where you wish in the class.) When you are finished, click on "Next."
You will then be asked to select the file(s) you wish to upload. You can either look for the files on your computer by clicking "Choose files," or you can drag and drop files from your computer into the "Drag files here" box. You can upload as many files to a file block as you wish.
Once you have selected the file(s) you want, click the "Upload" button at the bottom of the window.
Now, you have the option to write a description about these files if you wish to give your students context. You can also edit your file block title and add or remove files if you wish.
When you are ready, click the "Finish" button at the bottom of the window.