Are you using nginx in front of Phoenix? I had this problem once and found out nginx had a maximum size for post data and was rejecting them before they got to Phoenix. I had to google to find out how to raise that maximum limit in nginx but it fixed my situation.

For me the issue was on the js side, the elixir side was fine as it turned out, your form data might be incorrect.
What we did was that we appended everything to the formdata and only sent that like this for example:

function uploadAvatar(user, file, token) {
  let fd = new FormData();
  fd.append("user[avatar]", {
uri: file.path,
name: "test.jpg",
type: file.mime
  });
  fd.append("id", user.id);

  return {
types: [
  userConstants.UPLOAD_AVATAR_REQUEST,
  userConstants.UPLOAD_AVATAR_SUCCESS,
  userConstants.UPLOAD_AVATAR_FAILURE
],
callAPI: () =>
  axios
    .put(config.API_URL + "user", fd, {
      headers: { Authorization: "Bearer " + token }
    })
    .then(res => res.data)
  };
}

and you see where you append you need to set the key like it is in the example like “id” and if it is a param then you do like “user[avatar]” so your controller can use it to do what yo want to do

According to axios’ documentation JS part should be like this:

let data = new FormData();
data.append("recording[file]", rec);
data.append("recording[challenge_id]", 1);
data.append("recording[user_id]", 1);
data.append("recording[mod_score]", 3);
console.log(data);
axios.post(
    config.API_URL + "recordings",
    data,
    {
        headers: {
            Authorization: "Bearer " + this.props.auth.token,
            "Content-Type": "multipart/form-data"
        }
    }
);

To match your controller.