def upload_to_submission(self, file, user='<STR_LIT>', **kwargs):
user_id = obj_or_id(user, "<STR_LIT:user>", (User,))<EOL>return Uploader(<EOL>self._requester,<EOL>'<STR_LIT>'.format(<EOL>self.course_id,<EOL>self.id,<EOL>user_id<EOL>),<EOL>file,<EOL>**kwargs<EOL>).start()<EOL>
Upload a file to a submission.

:calls: `POST /api/v1/courses/:course_id/assignments/:assignment_id/ \
    submissions/:user_id/files \
<https://canvas.instructure.com/doc/api/submissions.html#method.submissions_api.create_file>`_

:param file: The file or path of the file to upload.
:type file: file or str
:param user: The object or ID of the related user, or 'self' for the
    current user. Defaults to 'self'.
:type user: :class:`canvasapi.user.User`, int, or str

:returns: True if the file uploaded successfully, False otherwise, \
            and the JSON response from the API.
:rtype: tuple