Why do I receive [object Object] in request Controller Vuejs+Laravel?

Question

const { isEmpty, data } = this.$refs.signaturePad.saveSignature();
console.log(isEmpty);
console.log(data);

let formData = new FormData();
formData.append('signature', this.$refs.signaturePad.saveSignature());

axios.post('/api/employee/update/27141399-8?api_token='+App.apiToken, formData, config)
.then(function (response) {
 currentObj.success = response.data.success;
})

When I do this:

  $request->signature;

I receive in my controller:

[object Object]

So I wonder where is the problem? if I add console.log to the signature it gives me this:

data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAABioAAAJxCAYAAADcoOoFAAAAAXNSR0IArs4c6QAAIABJREFUeF7s3c+LrUmaF/AYN25GqkXGndbIbEQYSxQUV+WAi9lIzT8g1S5ddbkQXAhVsxSErlq7qGpcupgqcCHCYNfGhQhjLcQfMM604CC6GAsFR8Fp+eL7jNGnT96b92bmiXif83khOXnvPXki4vNEZla93xMRPzNcBAgQIECAAAECBAgQIECAAAECBAgQIECAAIFFAj+zqF3NEiBAgAABAgQIECBAgAABAgQIECBAgAABAgSGoMIkIECAAAECBAgQIECAAAECBAgQIECAAAECBJYJCCqW0WuYAAECBAgQIECAAAECBAgQIECAAAECBAgQEFSYAwQIECBAgAABAgQIECBAgAABAgQIECBAgMAyAUHFMnoNEyBAgAABAgQIECBAgAABAgQIECBAgAABAoIKc4AAAQIECBAgQIA

Thanks