send-email-with-attachment-in-RN

You must save the image to device's picture directory.

import RNFS from "react-native-fs";

var destPath = RNFS.PicturesDirectoryPath + '/yourpicture.jpg';
RNFS.moveFile(data.uri, destPath)
.then((success) => {
console.log('file moved!');
})
.catch((err) => {
console.log("Error: " + err.message);
});

//then in your Mailer.mail attachment
//the correct path should be /storage/emulated/0/Pictures/yourpicture.jpg

path: destPath