Add ability to send file attachments to service

Problem/Motivation

* We need to be able to send file attachments through the service

Steps to reproduce

N/A

Proposed resolution

Surface required methods from https://docs.notifications.service.gov.uk/php.html#upload-your-file through the service.

Remaining tasks

TBC

User interface changes

N/A

API changes

N/A

Data model changes

N/A