files: File[];
variantData: FormData;
for (let i = 0; i < this.files.length; i++) {
   variantData.append('photos', this.files[i], this.files[i]['name']);
}

[
   [ File, File, File ],
   [ File ]
]

this.productGroup = this.fb.group({
  photosList: this.fb.array([
    this.fb.group({
      option: '',
      photos: ''
    })
  ])
})

options = { 'Red, 'Blue', 'Yellow' };
photos = [ ['imageRed1, 'imageRed2'], ['imageBlue1'], ['imageYellow1'] ];

const productSchema = mongoose.Schema({
  photos: [
    {
      option: String,
      photos: [String]
    }
  ]
});