Right now if you provide an Animated PNG (ANPG) file it will be treated as an image whether you set the asset type as image or video. Only the first frame will be rendered for the duration of the clip and there will be no animation.
We don’t have any immediate plans to support this however you can use QuickTime Mov files with alpha transparency. If you can convert or export your source animation to QuickTime format and use a codec that includes the transparency then you can acheive the same effect. Here is a guide on creating QuickTime animations with alpha channels: Create an overlay transition using After Effects — Shotstack.