There are file types that IIS will by default reject download attempts, typically this isn’t an issue as the rejected file types are Resource Type files such as .config, .vb, .vbproj, .mdf.

In all my years as a System Center admin, I have only run into two (2) file types that I normally have to Allow, that is .config and .mdf.

This is due to .Net applications or Access Applications not being packaged into CAB Files and having a <ExecutableName>.exe.config or Access Database contained within the Source Media.

It is easy enough to resolve this problem if you know where to look.  In IIS 7.5 (Windows Server 2008 R2) you can manage the Request Filtering right from within the UI.

On each of your Distribution Points, launch Internet Information Services (IIS) Manager and navigate to Sites | Default Web Site Under IIS category double click on Request Filtering. From here you will find the extension that is being blocked.  Right click on the extension and select Remove.

I normally add it back with an Allowed = True so that it is obvious that I made this change.