RadUpload control uses standard input type='file'
control. When the attach button is clicked a postback occurs and the file is uploaded. Information about the uploaded files is saved in the
variable. If you click send without attaching the file the following will be executed:
//Attach the files, uploaded with the current postback
CustomUploadedFileInfo fileInfo =
variable will be iterated.
The issue with your case is because the uploaded file is saved with a different name than the original (GUID). And the Attachment is sending the file with the temp file name.
Here is an example how to retrieve the file and set the original name :
//Attach the previously uploaded files
var test =
FileStream(info.TempFilePath, FileMode.Open, FileAccess.Read);
For more information please review the following help