Hello,
This is the AFNI forum, and the function that produced it is an FSL function. So you may want to ask that question over at their mailing list: [
www.jiscmail.ac.uk]
That said, what are you trying to open this file with? If it is an AFNI program, then it is not designed to load an FSL file. If you are using Matlab to open it (which is what your OS will default to if you click on it) then it will attempt to read the file as a binary blob, and it is actually plain text, so it won't load in matlab either.