[FromForm] use:     contentType: false

    [Frombody] use:      contentType: 'application/json; charset=utf-8',
 

  [HttpPost]
    [Route("[action]")]
    public IActionResult ToolOnchange([FromForm]string fab, [FromForm] string module, [FromForm] string tool){}

http://www.binaryintellect.net/articles/9db02aa1-c193-421e-94d0-926e440ed297.aspx