Modifies attachments of student submission. Attachments may only be added to student submissions belonging to course work objects with a workType of ASSIGNMENT. This request must be made by the Developer Console project of the OAuth client ID used to create the corresponding course work item. This method returns the following error codes: * PERMISSION_DENIED if the requesting user is not permitted to access the requested course or course work, if the user is not permitted to modify attachments on the requested student submission, or for access errors. * INVALID_ARGUMENT if the request is malformed. * NOT_FOUND if the requested course, course work, or student submission does not exist.