ho provato anche questa

strFileType = lcase(Right(strFileName, 4))

' Feel Free to Add Your Own Content-Types Here
Select Case strFileType
Case ".pdf"
ContentType = "application/pdf"
Case ".dxf"
ContentType = "application/dxf"
Case ".dwg"
ContentType = "application/dwg"
Case ".zip"
ContentType = "application/zip"
ContentType = "application/x-zip-compressed"
Case ".xls"
ContentType = "application/vnd.ms-excel"
Case ".doc"
ContentType = "application/msword"
Case ".wav"
ContentType = "audio/wav"
Case ".mp3"
ContentType = "audio/mpeg3"
Case ".mpg", "mpeg"
ContentType = "video/mpeg"
Case ".rtf"
ContentType = "application/rtf"
Case ".htm", "html"
ContentType = "text/html"
Case ".asp"
ContentType = "text/asp"
Case Else
'Handle All Other Files
ContentType = "application/octet-stream"
End Select

e sostituisco il Response.ContentType =