After enabling this plugin, I could no longer upload file types that WordPress previously allowed by default, until I added them myself to this plugin's list of allowed mimetypes.
This seemed counter-intuitive to me at first, but I do see how this could be desirable behavior for administrators. The plugin description reads, "allows users to define extra media types not allowed by the standard WordPress installation". This implies that the default file types should be sticking around.
So, this is either an issue, or at least a problem with the description.
Clean installation of WordPress, with only this plugin enabled.