At this moment Video creatives can not be hosted in the Adcombi technology. If the Adcombi technology is being used, the video creatives need to be hosted in the DSP (Adform) or an adserver (in case of DV360).
If you are going to use Adform, please read this page. In case you are using DV360, please follow their specifications which can be found here.
If publisher deals are being used, video creatives might have to be adapted to the publisher's specifications: