.dgn and .dtm MIME Types

I am having an issue accessing these file types when uploading Microstation files to our FTP site. 
I continue to get the following error:

We haver added the file extensions to the MIME Type list in IIS but the error persists. 

HTTP Error 404.3 - Not Found

The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.

Detailed Error Information
Module StaticFileModule
Notification ExecuteRequestHandler
Handler StaticFile
Error Code 0x80070032
Requested URL files.hwcengineering.com:80/.../CLA20.dgn
Physical Path W:\wwwroot\hwcfiles\Content\uploads\23\CLA20.dgn
Logon Method Anonymous
Logon User Anonymous
Most likely causes:
  • It is possible that a handler mapping is missing. By default, the static file handler processes all content.
  • The feature you are trying to use may not be installed.
  • The appropriate MIME map is not enabled for the Web site or application. (Warning: Do not create a MIME map for content that users should not download, such as .ASPX pages or .config files.)
  • If ASP.NET is not installed.