Given an array of bytes containing the file data, a MIME type, and a file name:

public void DownloadBytes( byte[] bytes, string mimeType, string fileName )
{
  Response.ContentType = mimeType;
  Response.AppendHeader( "Content-Disposition", "attachment;filename=" + fileName );
  Response.BinaryWrite( bytes );
  Response.Flush();
  Response.End();
}