mod:registerAssetId("test_img1.png", "TEST_IMAGE_1") -- without type specified, loads as a TEXTURE asset
mod:registerAssetId("test_img2.png", "TEST_IMAGE_2", "TEXTURE") -- loads as a TEXTURE asset
mod:registerAssetId("test_img3.png", "TEST_IMAGE_3", "ATLAS_CELL") -- loads as an ATLAS_CELL asset