post https://api.sandbox.uqpay.tech/api/v1/files/upload
Upload a file to UQPAY. The maximum size of the upload file is up to 20MB. Supported file types include jpeg, png, jpg, doc, docx, pdf. When you upload a file, the API responds with a file_id. This file ID can then be referenced as required as part of the attachments object in our other endpoints.