You can upload a variety of file formats (up to 50MB): images (.png, .jpg, .jpeg), videos (.mp4), documents (.pdf, .docx, .txt), spreadsheets (.xlsx), and presentations (.pptx).
To upload a file, click on the text field in your assignment submission form. This will reveal an "Advanced Settings" link.
Click this link to reveal the toolbar. Click on the image icon.
This will reveal the file upload instructions and walk you through the rest of the process.