There is likely a better way to do this, but what I did was go into the applicationhost.config file that IIS Express is using and find and comment out the mimeMap setting like this:
<!-- <mimeMap fileExtension=".m4v" mimeType="video/mp4" /> -->
<!-- <mimeMap fileExtension=".mp4" mimeType="video/mp4" /> -->
After making this change you can leave the web.config mimeMap settings and it will work on IIS7 and IIS8 Express.
If you need to know which applicationhost.config file IIS Express is using just right click on the IISExpress icon in the notification area icons (right side of your taskbar), then click "show all applications", then click the site name (not the URL) of the site you're developing for and it will show details about the site below, including the config file path. Sorry if this seems trivial, I had a tough time finding my config file as it ended up on a remote file server.