Almost all integrations in Mattermost send or receive payloads with a »application/json« content type. Incoming Webhooks, Outgoing Webhooks (configurable, see ticket https://github.com/mattermost/mattermost-server/issues/1649), Interactive Message Buttons… but not »Slash Commands«. Slash Commands are the only integration type, which sends it payload as »application/x-www-form-urlencoded«. Please allow sending a slash command with »application/json«.