In GetCourse mailings there is no way to attach a file or upload a video to a letter.
This is intentional, as any uploaded files add size to the message and negatively impact deliverability.
You can add a link to the file to the text of the letter (it can also be designed as a button with a link). The user will follow the link from the letter and download the attached file or examine it on the page.
So, here’s how to upload a file to the GetCourse file storage and add a link to a letter: