Hi,
The problem is the mime types are not registered on the server. If you look in the Web.config file, you'll find that there is a commented out <staticContent section that has the mime types. Uncommenting this section "may" solve the problem, but depending on other factors may also cause an error. I would try uncommenting it but be prepared to comment it back out if that does cause an error.
Note that in verison 2.3.8.5 there is one entry in that section like this:
<mimeMap fileExtension=".m4v" mimeType="video/m4v" />
subsequent to the 2.3.8.5 release one user has told me they needed to change that to:
<mimeMap fileExtension=".m4v" mimeType="video/mp4" />
If registering mime types by Web.config causes errors then comment it back out and look in your hosting control panel and see if they have something there that allows you to add mime types, if so you can add them there, if not you may need to contact your host for help in getitng those mime types registered.
On my server I was able to uncomment that section in Web.config and it solved the problem for me but others have reported errors when doing that, it may vary by operating system and security policy on the server whether you are allowed to register mime types by Web.config
Hope that helps,
Joe