BOPF Action exporting parameter not accepting variable length Strings for Base64 file

Hi,
I have a requirement to send a Base64 encoding data( converted from internal table ) to frontend using BOPF Action.
I tried using Variable length String in Exporting Type but i am facing error like Exporting parameter structure yyyyy_s of action xxxxx is not a flat structure .
Can anyone please help me in finding the correct way to send a String type to frontend using BOPF Action.

Thanks in Advance