Passing Byte data to a SAP FM

Can anyone help with the following:
We are trying to pass an attachment to a SAP FM.

I have a stream of byte data that I divide into a series of byte arrays of size 255.
This data then has to be passed to a SAP FM. The FM is expecting a collection of documents, each containing a string.

I have tried converting the byte arrays to strings and then assigning to the FM strings.
I have tried mapping the byte array objects directly to the FM strings.

The SAP Programmer is expecting the string to contain raw data in hexadecimal format.
I have tried converting the bytes to hexadecimal format and passing as strings and then as objects.

None of the above give the SAP programmer the same data as he gets from loading a file from disc.
Base 64 encoding is not an option as the SAP Programmer cannot decode it.

Any suggestions of how to progress this would be very welcome.