We were working on a web project that was served on IIS 6.  In the code, we were creating our own extensions (.BUYER, .PACK., .ITEM) for example.  The problem occurred when these files became unavailable for download when running the site through IIS. 

"Internet Explorer cannot download X"

The solution was simple. 

1. Open IIS, right click on the local computer name and select properties.

    2.   Select "MIME Types"

    3.   Select "NEW"  

    4.   Input the extension and application/octet-stream for MIME Type.  If you want to allow ALL extensions, add an asterick (*).

That simple!  Good luck :)