XPERIA C5 Ultra Dual video formats

[EUPHORIC2017]

Hi, there, who know 100% exactly which video formats possible to play on Sony's Video app? It's supported .mkv? When Sony's decided to not support white papers impossible to find trust information. In old white papers (first release) when wasn't updated to Marshmallow this model is mentioned only some formats like: m4, avi and ect. But i found in some not Sony's web sites what is supported HD format .mkv, it's true?

By the way why video which i can watch in for example K-Lite player is showing full screen in Video app is cutted screen top and down places, why? Video file is not converted, was converted to FHD 1920x1080 which is my model screen resolution, but result is the same. Why is cutting screen? Is not looks pretty when you watching. Is not cutting all video files, some movies is not cutted, The Simpsons is not cutted too. But just some files. All video which i tested, watched and ect was high HD resolution, most files is very high quality.

MODEL is: E5533 (Xperia C5 Ultra Dual) with Marshmallow ...129 firmware

Thanks in advance for answer