To restrict student submissions to specific file types go to the edit page of the assignment.

Under the Submission & Completion section there is an Allowable File Extensions dropdown.

You can either select one of the available options but if you want to restrict the files further, select Custom File Types. This will give you an additional pop-up box where you can add the file types you want to allow.

The Allowable Filetypes field provides options for the types of files that learners can submit for the assignment. 

• Annotatable files - includes all file types that are supported by the Annotation tool in Assignments.
• .PDF only
• Files that can be previewed without any conversion - Including .HTM, .HTML, .MHT, .MHTML, and .BMP
• Images and videos
• Custom filetypes