Uploading files displays 'cannot upload this file type'

The default WordPress behaviour will only allow files of specified file-types to be uploaded. If a file is uploaded that is not in the list of permitted file types, WordPress will display the error 'Sorry, you are not allowed to upload this file type'.

What is the default allowed list?

jpg jpeg png gif mov avi mpg 3gp 3g2 midi mid pdf doc ppt odt pptx docx pps ppsx xls xlsx key mp3 ogg flac m4a wav mp4 m4v webm ogv flv webp

Can this list be changed?

WordPress allows you to add file types to the above list. To do this, go to the Network Admin of your site, then go to 'Settings'. Find the field for "Upload file types". Add the file types you wish to upload to the list. Once you save the settings, you'll be able to freely upload files with the newly added file extension.