It’s just because you’re not using the extensions given in your available array 
Here using your .ktx & .ktx2 files:
https://playground.babylonjs.com/#8S4DM1#3
[edit]
Hmm, it seems only .jpg are loaded
I’m not sure but maybe you should keep ktx name convention (['-astc.ktx', '-dxt.ktx', '-pvrtc.ktx', '-etc2.ktx', '-etc1.ktx'] - and keeping only available formats in this list) and rename your files?