Cannot upload pptx files to sessions

When creating a new session a user is unable to upload pptx slides.

Steps to reproduce.

1) Create a new session (content -> add content -> session)
2) Go to the section for "slides"
3) Attempt to upload pptx file
4) You will get the following error message "The specified file Cod Fish.pptx could not be uploaded. Only files with the following extensions are allowed: ppt pdf odt odp."