This is feature request. Please add ability to use .webp as video “Cover art”
How well is this supported by various software? A brief search on my end says not really at all. If we let you and something does not work will you report it as a bug in Shotcut?
Matroska is one of the most forgiving containers out there, and even it supports only JPEG and PNG officially. Agree, WebP would be ignored or considered malformed by most players.