Testing in nestjs multer file upload

Question

I have written below code in nestjs.

export const multerOptions = {
        storage: diskStorage({
            destination: './uploads',
            filename(_, file, callback) {
            const randomName = Array(32)
                    .fill(null)
                    .map(() => Math.round(Math.random() * 16).toString(16))
                    .join('');
                return callback(null, `${randomName}${extname(file.originalname)}`);
            },
        }),
    }

when I am testing it is using below code.

  it('should pick the file', () => {
        const cb = jest.fn();
        expect(multerOptions.storage).toBeTruthy();
        expect(cb).toHaveBeenCalled();
        expect(cb).toHaveBeenCalledTimes(1);
        expect(cb()).toBeFalsy();
    });

It is saying below lines of code is not covered.

            const randomName = Array(32)
                    .fill(null)
                    .map(() => Math.round(Math.random() * 16).toString(16))
                    .join('');
                return callback(null, `${randomName}${extname(file.originalname)}`);

what extra I need to include in my test case to cover these lines?