Slack's Interactive Messages endpoint reports a Content-Type of `application/x-www-form-urlencoded` but responds with something like looks like `payload=%7B%22actions%22%3A%5B%7B%22...`, which is encoded JSON inside the payload parameter, which takes the API Gateway VTL templates for a loop.

slack-interactive-messages-api-gateway-mapping-template.vtl

## The `substring(8)` returns the input with everything after "payload=" which is the only parameter passed in,
## everything else is encoded JSON inside that parameter.
#set ($encodedJSON = $input.body.substring(8))
$util.urlDecode(${encodedJSON})

Saved my day. Thanks.