assetbundle in Unity 5.6 can't load png files?

I'm making an assetbundle of jpeg and png files in Ubuntu. I was able to package it inside an assetbundle but strangely while loading it back from the bundle, the png images won't load. I have no problem with jpeg files but the png files just won't load and shows null. I can see a lot of people using png files in the internet.

OS Ubuntu 16.04 - Unity 5.6.1xf1.

AssetBundleRequest req = bundle.LoadAssetAsync("Assets/file2.png"); yield return req; Texture model = req.asset as Texture; if(model == null) { Debug.Log("Null object"); } If I replace the above snippet with a jpeg file on the bundle it works fine. Can anyone help? I'm pretty new to unity.