Thank you for contacting Syncfusion support.
We have checked your sample; it seems that you have done namespace changes only in the import.ashx.cs file. We request you to change the namespace in the import.ashx file also. Kindly have a look at the following code example.
<%@ WebHandler Language="C#" CodeBehind="Import.ashx.cs" Class="DocumentExample.Import"%>
Refer to the following screenshot: