It probably too late now, but for others who get that problem here is how to solve it:
I solved it for mp4 videos but it is the same principal for any mime,
just revise to your needs.
I assume you use vs2012, create IHttpHandler and copy this code into it:
public class Mp4Handler : IHttpHandler
{
public void ProcessRequest(HttpContext context)
{
context.Response.ContentType = "video/mp4";
context.Response.BinaryWrite(File.ReadAllBytes(context.request.PhysicalPath));
context.Response.End();
}
public bool IsReusable{ get{ return false;}}
}
And remember to add to your web.config file under system.web:
<httpHandlers>
<add verb="*" path="*.mp4" type="[your-namespace].Mp4Handler" />
</httpHandlers>
By doing so you will not need CassiniDev to serve mp4 correctly anymore, however,
CassiniDev is not evil and worth keeping - without it I wouldn't be able to verify what the problem was in the 1st place.