A little background why this error happens. Strings in the xUML Runtime are Unicode strings represented in UTF-8 and a zero byte is the end of string marker.
The FileSystem adapter can read the data because it will return a blob. But then converting the blob to string does not work because it stops at the zero byte.
I have no ready solution for replacing the zero byte in the blob. Perhaps an intermediate step converting it to base64 or hex and replacing the zero byte is possible.