Assigning proper type for schema field that is to store Node.js buffer array (saving decoded base64 JSON image data)

Hi everyone.
My situation is the following:
I’m integrating OpenAI’s DALLE-2 API into my web app. The generated picture is returned as a base64-encoded json string, which I am converting into Node.js buffer type:

function decodeBase64JsonAndSaveAsBuffer(b64jsonString) {
    let binData = Buffer.from(b64jsonString, 'base64');
    return binData;

The return from this function is saved as Mongoose schema type Buffer in my image’s schema/model. I think I have all this right up to that point. Now, however, GraphQL complains that the String type I have assigned to the binData field for my Image type can’t represent a buffer array:

Ok, I understand I need to change the type for this field. The question is, to what do I change it in light of I need to handle this Nodejs buffer array input?