Return the registered filesystem for the given file.

def get_filesystem(filename):
    """Return the registered filesystem for the given file."""
    filename = compat.as_str_any(filename)
    prefix = ""
    index = filename.find("://")
    if index >= 0:
        prefix = filename[:index]
    fs = _REGISTERED_FILESYSTEMS.get(prefix, None)
    if fs is None:
        raise ValueError("No recognized filesystem for prefix %s" % prefix)
    return fs