Try to be the unpacker. Usually you would need some kind of information about the file you need to unpack at a fixed file position. Like the start, or the end.
Try to indentify obvious breaks in the file, and correlate those with numbers found at the start or end of file.
But, maybe the file is compressed without any header, or encrypted.
Sometimes it helps to find the binary which does the decoding, and try to identify the relevant code in it. Maybe you will find a string hinting at a specific compression algorithm, or a decryption key.
Try googling for the value of the first few bytes of the file, maybe someone else posted about it.