Hi Glen, 

Per our ticket, I was able to replicate this error after I made a change in the settings.  As the system administrator I went to Settings > Configuration > Security > Uploads tab.  I removed the default asterisk (.*), that allows all file types, from the top section "Allowed extensions for document uploads in HTML editor" (see the below image).  In that text box I put ".doc" (no quotes), then tried to upload a .png to a kb article.  That is when I got your error.