During Assignment configuration, the examples provided next to the "Restrict Upload File Types" are "doc,ppt,txt". Some of our less tech-savvy faculty have used the example to help them specify their file type restrictions, not realizing that the modern (Office 2007 and later) formats are post-fixed with an 'x' (docx, pptx). This has in turn driven students to reach out to our IT department asking why they are unable to upload their modern Word, PowerPoint, etc. documents.
My suggestion would be to update this example text to reflect the modern file formats (i.e. "docx,pptx,txt").