attach a file from a mobile app to an e-mail

In my mobile app, I need to figure out a way to take a file and attach it to an e-mail. This app shows it's possible - about half way down the page is their documentation to show users how (see section called "iOS & Android: Sending a file"). I looked up sending attachments using mailto and quickly found out that's not possible. This app is built using Expo, Vue Native and some React Native. How can this be achieved?