'Load a key from a file.
@param filename: The path to load key data from.
@type type: L{str} or L{None}
@param type: A string describing the format the key data is in, or
L{None} to attempt detection of the type.
@type passphrase: L{bytes} or L{None}
@param passphrase: The passphrase the key is encrypted with, or L{None}
if there is no encryption.
@rtype: L{Key}
@return: The loaded key.'

@classmethod
def fromFile(cls, filename, type=None, passphrase=None):

    with open(filename, 'rb') as f:
        return cls.fromString(f.read(), type, passphrase)
