// img
// {
//   uri: '"assets-library://asset/asset.JPG?id=SOME_ID&ext=JPG"',
//   md5: "some md5"
// }

// copy into cache dir first
const path = FileSystem.cacheDirectory + img.md5
await FileSystem.copyAsync({
  from: img.uri,
  to: path
})

// read that and do upload
const file = await FileSystem.readAsStringAsync(path, {
  encoding: FileSystem.EncodingTypes.Base64
})

const payload = {
  title: 'some awesome photo',
  image: file
}

fetch('somewhere', {
  method: 'POST',
  body: JSON.stringify(payload)
})