Some assignments require you to submit more than one file at the same time, e.g. a separate works cited document.
To do this, you must first have the files saved to your device's storage. If you are using the Student Portal / web version of Office 365, you can do this by selecting File > Save As > Download a Copy.
It'll ask you if you want to download the file, then will download it to your device's Downloads folder, unless otherwise specified.
You'll then have to use the File Upload function on your assignment, after hitting Submit Assignment - you cannot submit multiple files with Office 365 or Google Drive.
Click on the Choose File button to select your first file, and then press the Add Another File link to add another Choose File box, selecting your files one at a time until all of the files you want to upload are selected. Once you're finished, click Submit Assignment.