Hi Szymon,
the easiest way would be to leave the file outbound in UNICODE (UTF-8).
UNICODE should be able to display all needed characters and B1i's internal processing is in UNICODE (UTF-8) as well.
If you decide to change the encoding, B1i's the outbound needs to be converted from UTF-8.
You can only select the encodings listed in the drop-downbox for file outbound definitions.
Please select the one whichs fits and test it carefully.
Best regards
Bastian
P.S.: the easiest way to work with file adapter in B1i is to use unicode utf-8 for file inbound and file outbound. Just make sure that your 3rd party application writes/reads UNICode utf-8 as well.