Check if this discussion may be of help
http://stackoverflow.com/questions/146134/how-to-remove-illegal-characters-from-path-and-filenames

>If even try to Decode the file name and plug the mime type, such as:
>
>

>            ' Get the file length
>            oProcess.oResponse.Clear()
>            oProcess.oResponse.ClearContent()
>            oProcess.oResponse.ClearHeaders()
>            oProcess.oResponse.Charset = cCharset
>            'oProcess.oResponse.ContentType = cContentType
>            oProcess.oResponse.ContentType = GetMimeType(lcFileName)
>            oProcess.oResponse.AddHeader("Content-Disposition", "attachment;filename=""" + oApp.Decode(lcFileName) + """")
>            oProcess.oResponse.AddHeader("Content-Length", lnSize.ToString())
>            oProcess.oResponse.ContentEncoding = System.Text.Encoding.GetEncoding(cContentEncoding)
>
>            oProcess.oResponse.TransmitFile(cFile)
>
>            ' Make sure to end this process as we do not want any additional process to occur
>            oProcess.oResponse.End()
>
>            Return True
>        End Function
>
>        Private Function GetMimeType(fileName As String) As String
>            Dim mimeType As String = "application/unknown"
>            Dim ext As String = System.IO.Path.GetExtension(fileName).ToLower()
>            Dim regKey As Microsoft.Win32.RegistryKey = Microsoft.Win32.Registry.ClassesRoot.OpenSubKey(ext)
>            If regKey IsNot Nothing AndAlso regKey.GetValue("Content Type") IsNot Nothing Then
>                mimeType = regKey.GetValue("Content Type").ToString()
>            End If
>            Return mimeType
>        End Function
>

>
>But, that generates the "Illegal characters in path." at the TransmitFile() line and the browser runs indefinitely waiting for something.