webix.ajax().headers({
"Content-type": "application/json"
})["post"](url, params, ajaxCallback);
// when the mode is post, Content-type will be replaced by 'application/x-www-form-urlencoded'
I can confirm the issue.
Fix will be included in the next build ( custom content-type will not be overridden anymore) Fixed build will be available till the end of this week.
Thanks.