I do not think that WinSCP adds that. Did you try uploading the file using any other SFTP client? It might be at their end.
If you want to us to investigate further, please attach a full session log file on Debug 2 log level.
is UTF-16 Byte-order mark (BOM):
Actually, isn't it the other way around? The BOM might actually be in your source file (it's not visible in your editor – the editor adds it while saving the file). It might be the email client that strips the BOM when sending (or when receiving at their end).