Vendor Documentation
myCourses restricts certain mime-types for files across myCourse for system security. Those mime-types are outlined in the vendor's documentation at the link below. These are primarily program and programming related file-types.
If you need to submit a file of this type you will have to compress the file into a ZIP archive before making the submission.
Turnitin Restrictions
If an Assignment is using the Turnitin integration there will be additional restrictions on accepted file submissions. Turnitin only accepts a limited number of file types for evaluation and has additional size and document length restrictions.