Upload one receipt image and attach it to a transaction entry. Send the image as multipart/form-data in a field named file; a JSON or base64 body is not accepted. Only JPEG and PNG images are supported, up to 10 MB each. The image is stored as uploaded, without resizing. A transaction entry can have at most 10 attachments, counting those added in the Hostex web app; when it already has 10 or more, the request is rejected with 400 and existing attachments are left unchanged. To attach several images, call this endpoint once per image. Uploads are limited to 60 per account every 10 minutes. Adding an attachment does not change the entry's updated_at and does not send a webhook. Returns 404 if the entry does not exist or is not accessible to the current operator. Requires a token with write access.