As I don't have the corresponding archive keys of my attached files (EH GUID is not enough) I did not figure out how to use /SAPTRX/BAPI_EH_GET_FILES.
Nevertheless it's code revealed the attached file header table. I tracked the entries of my files send via BAPI and realised I forgot to set the <Filesize> attribute.
So I marked your answer as correct, it led to the final solution. Thanks very much 