The POST /api/v1/merchant/attachment API method allows users to add attachments to a merchant's application in the payment processing service. This endpoint requires authentication and accepts file attachments, such as documents or images, related to the merchant’s application. The request body must include the necessary file data and metadata. The response returns a status code indicating whether the attachment was successfully uploaded or if there were any errors. This method is essential for submitting supporting documents during the merchant onboarding process.