post_type shouldn’t be needed. If it has a topic_id it knows it is a post and not a new topic. This is how I’m doing it with curl:
curl -i -sS -X POST "http://localhost:3000/posts.json" \
-H "Content-Type: multipart/form-data;" \
-H "Api-Key: 079fb2bb12d3b436bb11bde8eb58aaa9a36560fa7d79b14b3087aa40b1ebc2c4" \
-H "Api-Username: blake.erickson" \
-F "raw=92d2e4a938f8d2c65e3fbbcf68e4c272 374f11b3488a03d5d299e862003b09a3 76cdef8f63901f150f2bbf5579cd0b22" \
-F "topic_id=11" \
-F "whisper=true"
HTTP/1.1 200 OK
