I have a VideoBlock with one property:

[ContentType(
    DisplayName = "Video block",
    GUID = "1C9DB61E-D5D8-47A6-A7B4-4780146A05C0")]
public class VideoBlock : NctBaseBlock
{
    [Display(
        Name = "Video",
        GroupName = SystemTabNames.Content,
        Order = 10)]
    [UIHint(UIHint.Video)]
    public virtual ContentReference Video { get; set; }
}

When editing this property the content selector displays only the root note (the "For this site" folder - see this image). If I change the UIHint to UIHint.MediaFile, the full tree is displayed with all of its files (see this image). The workaround now is to leave it as UIHint.MediaFile and let editors potentially select other file types than videos. I am aware that editors are able to drag video files directly onto a ContentArea, but in cases where I want the video as one of many properties in a block I need the editor to be able to pick it from a content selector.

I have verified that this happens in the Alloy MVC templates for EPiServer 7.5 as well. It works for UIHint.Image, so it seems to be related to videos. It also can't be the case that the content selector doesn't find any video files, because it still shows the whole folder tree when the UIHint is UIHint.Image and no images exists.