FormData can't be inspected from the web developer console. You can only use them to create key value pairs to be sent.
FormData
If you want to debug it, you may do this,
for (var p of formData) { console.log(p);}
formData.getAll('keyName') will return an array of all the values stored in a specific key. This can be viewed from your Web Dev Console.
formData.getAll('keyName')