CSP - Download file, manipulate and return as attachment

%CSP.StreamServer is just a helper to cut short some manual labor.

What you need to do is write the file stream back to the device and change three Content-* headers as follows.

Your CSP page has an OnPreHTTP just like every %CSP.Page extended class. You can use this method to modify the Content-Type, Content-Disposition and Content-Length headers. In your case you'll need to use the <script> tag syntax for creating the method.

The example below assumes you're using a class instead.

NOTE: If you really customize your %CSP.Page or %CSP.REST. You don't even need to use the OnPreHTTP method. But I'll be using it here for educational purposes.

ClassMethod OnPreHTTP() As %Boolean [ ServerOnly = 1 ]
{
   set file = %request.GetMimeData("FileStream")
   if '$isobject(file) return 0
   
   do ##class(%CSP.StreamServer).FileClassify("XLS", .contentType, .isBinary, .charset)
   do %response.SetHeader("Content-Type", contentType)
   do %response.SetHeader("Content-Disposition", "attachment; filename=""_file.FileName_""")

  return 1
}

ClassMethod OnPage() As %Status [ ServerOnly = 1 ]
{
  set iStream = %request.GetMimeData("FileStream")
  $$$QuitOnError(##class(something).method(iStream, .oStream))
  
  do %response.SetHeader("Content-Length", oStream.Size)
  
  do oStream.Rewind()
  do oStream.OutputToDevice()
  
  return $$$OK
}