Parsing oscilloscope binary data
This seems pretty nice. I have just some nitpicks about this part:


postfixs = [".isf", ".ISF"]
if os.path.splitext(binaryfile)[-1] not in postfixs:
    raise ValueError("File type unkown.")



Nitpicks:


I generally don't like repeating the same thing in lowercase and uppercase versions. I prefer to use lowercase, and convert untrusted input to lowercase. However, that will make this check less strict: iSF and ISf also become valid extensions, which you might not want.
I'd spell out "postfixs" as "postfixes", or better yet, use the more common term "extensions"
Instead of a list of allowed extensions, I'd use a set. Although in this example you surely won't be hitting performance issues, it's a matter of principle and habit


So I'd write this as:

extensions = set([".isf"])
if os.path.splitext(binaryfile)[-1].lower() not in extensions:
    raise ValueError("File type unknown.")


As for the rest, this seems pretty clean to me. I especially like the assert statements documenting your assumptions.