Canvas allows files to be attached to discussion posts, however that option is not the default. If you want your students to be able to upload attachments to their discussion posts, you simply need to enable the “Let Students attach files to Discussion Posts” option in your course settings.
Here are the steps:
1. Open your Canvas course
2. Click on “Settings” in the left side course navigation
3. Click “Edit Course Details”
4. Click “More options” at the bottom
5. Check the box next to “Let Students attach files to Discussion Posts”
6. Click “Update Course Details”