From your question - I presume that you don't know how to decode the data, because you don't know how it has been encoded?
There exists a utility known as 'file magic' which tries to identify the format/encoding for any file.
If you had a unix system to hand, you would , simply type.
'file filename'
alternatively- if you do not have the 'file' utility easily available, and the file is not too huge, you could send it to me and I will quickly try that for you.
https://en.wikipedia.org/wiki/File_%28command%29
