javascript unit testing: I need to change the returned value of a called function in the tested function

I want to test the type of uploaded file, but since it doesn't need to be uploaded I just want to change the returned value of the function that gets the type

I read about stubs but I couldn't use expect after stubbing, I think I'm getting things wrong I want to test that function and change the returned value of fileType

const check_avatar_errors = avatar => {
  const allowed_types = ["image/png", "image/jpg", "image/jpeg"];
  const file_type = fileType(avatar.data).mime
  if (!allowed_types.includes(file_type)) {
    return create_error("invalid_content_type");
  }
};

so That the next test would pass :

const validators = require("../../helpers/validator.js");
const check_avatar_errors = validator.__get__("check_avatar_errors");

    it("should return invalid content type error if the uploaded file is not an img",function () {
   /************** Some code here ********/
      expect(check_avatar_errors("not_img_type")).to.deep.equal([{error: "invalid_content_type"}]);

    });

Thanks in advance