let options = {
  uri: '/asset/v1/content/assets',
  json: true,
  body: MCData
}

const RestClient = new FuelRest({auth: FuelAuthClient});
RestClient.post(options)
.then((result) => {
  console.dir(result);
})
.catch((error) => {
  console.dir(error);
})

{
  "name":"Document 123",
  "assetType":{
    "name":"png",
    "id":28
  },
  "file":"data:image/png;base64, REST OF THE Encoded IMAGE HERE"
}

{ message: 'Internal Server Error',
 errorcode: 0,
 documentation: 'https://developer.salesforce.com/docs/atlas.en-us.mc-apis.meta/mc-apis/error-handling.htm' }