User.ReceiveFile: Add a shared file to personal file namespace
==============================================================

.. function:: ReceiveFile(filename string, sender string, accessToken UUID) (error)

  Adds a file that was previously shared with the caller (i.e. the recipient) to
  the personal file namespace of the caller.

  Returns an error if:

  - the secure file share invitation pointed to by ``accessToken`` has been
    revoked; or

  - the recipient already has a file with the given ``filename`` in their
    personal file namespace; or

  - the recipient is unable to verify that the secure file share invitation
    pointed to by the given ``accessToken`` was sent by the given ``sender``; or

  - the recipient is unable to verify the integrity of the secure file share
    invitation pointed to by the given ``accessToken``.

  :param filename: The filename that the recipient would like to assign to the
                   shared file in their personal file namespace.
  :type filename: string
  :param sender: The username of the sender.
  :type sender: string
  :param accessToken: The UUID storage key at which the sender's secure file
                       share invitation is stored in the
                       :doc:`/servers/datastore`.
  :type accessToken: UUID

  :rtype: error

.. warning::
    Do not forget that your design must satisfy **all** requirements, including:

    #. There must only be a single copy of the file (see
       :ref:`requirement_sharing_and_revoking` #2).

    #. All :ref:`requirement_access_tokens` requirements.

    #. Users can have multiple active user sessions at once (see
       :ref:`requirement_user_sessions`).