thank you for quick answer, have a nice day.
there is no fix possible - filename transmission is not part of the OFTP2 protocol.
Please do never rely on filenames in EDI transmissions (e.g. reference no in filenames or something like that). This is mainly not supported by EDI transmission protocols like x.400 (ok - it's possible in FTAM from 1988), AS2 (only with extensions) and OFTP2.