When you want to use files in your assignment, for example images or pdf documents, you can attach these to the assignment. Such files can be displayed, but you can also allow the students to download the files.
To attach a file, go to the assignment editor, and then select FIles in the left panel. Here you see the list of files attached to the assignment. Initially the list is empty.
Press the button Upload File to upload a file. You are shown a file selector where you can pick the file to upload. The file is now added to the list. Note that the file gets a label. This is determined by the file name, but can be changed. The label is used when displaying the file or when creating a download button for the students.
If you upload an image (.png or .jpg) you can select a part of the image.
In the list of files, at the right, there are two buttons. With the first you can edit the label of the file. And with the second you can delete the file.
Note that, when you copy an assignment (or a module or course that contains the assignment) also the attached files are copied.