Tengo el siguiente código, implementado en un sitio https Asp, compilado con MVC 4.0:

public FileResult ANotSoWorkingFunction(string filePath, string fileName)
{
 pathToFile = string.Format("~/{0}/{1}", pathToFile, fileName);
 return File(new FileStream(pathToFile, FileMode.Open), "application/pdf", fileName);
}

Esto funcionará (como muchos de ustedes probablemente ya hayan adivinado) con Chrome, Firefox e IE9. Pero arrojará un:

---------------------------
Windows Internet Explorer
---------------------------
Internet Explorer cannot download someFileName from a_site.com.


Internet Explorer was not able to open this Internet site.  The requested site is either unavailable or cannot be found.  Please try again later.
---------------------------
OK   
---------------------------

On IE6,7,8

Cualquier idea o pista sobre esto es muy apreciada, ya que me paso el día jugando con el encabezado html.

EDITAR

Aquí está el encabezado de IE7:

HTTP/1.1 200 OK
Cache-Control: private, no-cache="Set-Cookie"
Content-Type: application/pdf
Server: Microsoft-IIS/7.5
X-AspNetMvc-Version: 4.0
X-AspNet-Version: 4.0.30319
Set-Cookie: .ASPXAUTH=; expires=Mon, 11-Oct-1999 21:00:00 GMT; path=/; HttpOnly
X-Powered-By: ASP.NET
Date: Wed, 04 Apr 2012 08:43:50 GMT
Content-Length: 233324

Y aquí están los de IE9:

HTTP/1.1 200 OK
Cache-Control: private, no-cache="Set-Cookie"
Content-Type: application/pdf
Server: Microsoft-IIS/7.5
X-AspNetMvc-Version: 4.0
X-AspNet-Version: 4.0.30319
Set-Cookie: .ASPXAUTH=; expires=Mon, 11-Oct-1999 21:00:00 GMT; path=/; HttpOnly
X-Powered-By: ASP.NET
Date: Wed, 04 Apr 2012 08:42:14 GMT
Content-Length: 233324

Gracias