Display a helpful message for file types that we don't know how to display

[benbc]

For any output that contains a file of a type that we don't know how to display, the bottom of the output detail pane should display a message saying "This type of file cannot be displayed. It should be reviewed outside this application."

Notes:

• There is a separate later issue coming to allow output checkers to open the file in another application from within the viewer.
• This should work for files of any type, including those that we've never considered or heard of before.
• This is related to, but separate from, #118.