The API will respond with a 400 if you are sending a rich push but you don't limit the device_types to platforms that are compatible with rich push. These incompatible platforms include wns, mpns, and blackberry.
{
"notification": {
"alert": "Just one"
},
"message": {
"title": "The title, device registered to 1234567890 only",
"body": "<html>The body</html>",
"content_type": "text/html"
},
"audience": {
"alias": "01b307acba4f54f55aafc33bb06bbbf6ca803e9a"
},
"device_types": ["ios","android"]
}