This thread seems to be a duplicate of the following threads from that all contain mostly the same problem:

• Issue with Arc on Ubuntu - #4 by NobbZ
• Plug.upload doesn't seem to pick up uploaded file from app but works from web interface
• Questions about ARC in phoenix/elixir api
• How to send a picture in json as from web to elixir

Please check if any of them contain an answer for you. Really not sure why there are so many threads about this, with the same filenames even. (In fact this is a verbatim copy of the last thread in that list.)

Do note that for large pictures, this will be quite a large string, since base64 makes everything roughly 33% larger IIRC.

Have you tried FormData?

export function uploadImage(file) { // so uri, type, name are required properties
  const formData = new FormData();
  formData.append('image', file);

  return fetch(`${imagePathPrefix}/upload`, { // give something like https://xx.yy.zz/upload/whatever
    method: 'POST',
    body: formData,
  }
  ).then(
    response => response.json()
  ).then(data => ({
    uri: data.uri,
    filename: data.filename,
  })
  ).catch(
    error => console.log('uploadImage error:', error)
  );
}

https://github.com/facebook/react-native/blob/master/Libraries/Network/FormData.js