The FileManager is not showing icons for video files, e.g. mp4. The icon's CSS class is k-i-video, but it may need to be k-i-file-video.

The workaround is to apply the k-i-file-video icon to k-i-video:

CSS

.k-i-video::before {
  content: "\e93b";
}

If there are many subfolders below a folder the list becomes too long for the TreeView window and gets cut off at the bottom. Apparently, there are some height and overflow settings missing. After some digging around in the rendered HTML and copying some CSS selectors (using F12 on Edge) I got it working with this in my CSS file:

.k-filemanager-content-container > div > div.k-pane.k-filemanager-navigation.k-pane-static > div > div > ul {
    height: 100%;
    overflow: auto;
}
.k-filemanager-content-container > div > div.k-pane.k-filemanager-navigation.k-pane-static > div > div {
    height: 100%;
}

----------ADMIN EDIT----------

In the meantime, here is a possible workaround:

<style>
 .k-treeview-lines {
        height: 100%;
 overflow: auto;
    }

    .k-filemanager-treeview {
        height: 100%;
 }
</style>