更改您的JS参数DocumentId以匹配您的控制器ID.

Change your JS param DocumentId to match your controller id.

$.ajax({
  type: "GET",
  //"Document/GetByReview?reviewId=" + that.selectedReview.ReviewId;
  url: constants.serviceUrl + "Document/GetFileContents?id=" + docId,
  contentType: "application/json; charset=utf8",
});