Uploads receipt files to a transaction's expense. Send them as multipart/form-data in the files field. The identifier can be the card transaction ID (CTX-...) or its expense ID (EXP-...).

Accepted formats are PNG, JPG/JPEG, HEIC, WebP, and PDF, up to 10 MB per file and 5 files per request. If any file fails these checks, nothing is uploaded and the call returns 422 with UNSUPPORTED_FILE_TYPE, FILE_TOO_LARGE, or TOO_MANY_FILES. A HEIC file is also stored as a JPEG copy, so it adds two receipts.

The response is {"success": true} with the updated transaction in data, in the same shape as Fetch expense transaction details, including the new receipts. Uploading can also move an incomplete expense to pending_review once its required fields are filled. To remove a receipt, use Delete expense receipt.