When trying to download Wix*.msi from http://wix.sourceforge.net/releases the wrong MIME type is sent by the web site (text/plain). Firefox will download it as such, while IE sniffs the content and downloads it correctly as a binary stream. IE seems to be the only (major) browser that may ignore the Content-Type header and sniffs for MIME type.
This is a regression, likely happening around the time SF stopped showing directories by default. You can add "application/x-ole-storage msi" to your mime.types if you have access, or "AddType application/x-ole-storage .msi" to your apache config if you have access.
Alternatively you could just use MIME type application/octet-stream since it really only needs to be interpreted as a binary stream.