const payload = {
    query: `
        mutation($file: Upload!) {
            server:createServer(
                input: {
                    bannerFile: $file,
                    name: "${values.serverName}",
                    address: "${values.serverAddress}",
                    description: "${values.serverDescription}"
                }) { id }
            }`,
    variables: {
        file: values.bannerFile // this comes from the input[file]
    }
};

superagent.post('/api').send(payload).set('accept', 'json').then(result => {
    // handle response
});