The pop-up validates before uploading: you must pick a supplier ("Please select a supplier before importing.") and a file ("Please upload a receipt image before importing."). Files over 20 MB show "File is too large. Please upload an image under 20MB." and non PNG/JPG/JPEG files (including HEIC from iPhones) show "Please upload a valid image file...". The server repeats these checks ("Invalid file type. Please upload PNG, JPG, or JPEG.", "Please select a supplier.") and reports upload problems such as exceeding the server upload limit. Images are resized to 1600 px before analysis, so a normal phone photo is fine.