ACTIVE SERVER PAGE

  'values  
base64String="data:image/gif;base64,R0lGODdhMQAiAPcAAP////f39+/v7+fn597e3s7OzsbGxr29vbW......"


Set tmpDoc = Server.CreateObject("MSXML2.DomDocument")
Set nodeB64 = tmpDoc.CreateElement("b64")
nodeB64.DataType = "bin.base64"
nodeB64.Text = Mid(base64String, InStr(base64String, ",") + 1)

With Response
.Clear
.ContentType = "image/gif"
.AddHeader "Content-Disposition", "attachment; filename=testImage.gif"
.BinaryWrite nodeB64.NodeTypedValue 'get bytes and write
.End
End With