Hittade lösningen själv.
Alla content-types finns lagrade i respektive maskins register under HKEY_CLASSES_ROOT
Lite ful kod för att visa hur man kan utläsa detta från registret:
<%@ Page Language="VB" %>
<%@ import Namespace="System" %>
<%@ import Namespace="Microsoft.Win32" %>
<script runat="server">
Public Sub Page_Load(Sender As Object, E As EventArgs)
Dim _extension As String = ".pdf"
Dim _regKey As RegistryKey
Try
_regKey = Registry.ClassesRoot.OpenSubKey(_extension)
litReg.Text = "Key: " & _extension & "<br>" & "Value: " & _regKey.GetValue("Content Type")
Catch Ex As Exception
litReg.Text = "<font color=""#FF0000"">" & Ex.Message & "</font>"
End Try
End Sub
</script>
<html>
<head>
</head>
<body>
<form runat="server">
<asp:Literal id="litReg" runat="server"></asp:Literal>
</form>
</body>
</html>
cya,
/PatrikB