This may come as some surprise to you, but Chrome OS doesn't currently have a dedicated media player app. Viewing photos and videos is done through the Files apps, which works fine for the most part. It appears Google is working on a media player app for Chrome OS and it will be a System Web App (SWA).
The latest version of Chrome OS Canary has a rudimentary media app that can be accessed from chrome://media-app along with a shortcut in the launcher. Right now, the app consists of a simple dialog that says you can "Drag and drop a file or select open" and an "OPEN" button. Videos play just fine in the app, though there aren't many controls. Photos appear with a few basic image editing options. There's not much to it.
As mentioned, this is a System Web App just like the existing Settings app. That means you can access it from a URL in the regular browser window, but it also comes with an app icon in the launcher. Google has also been working on a System Web App version of the Camera, which can be enabled in the Canary version of Chrome OS with the flag #camera-system-web-app. Google may be in the process of converting all Chrome OS apps to System Web Apps.
Source: ChromeStory