Ajax Control Toolkit HtmlEditorExtender insert image doesn't work - asp.net

Question

When i try to upload an image firstly it says (Uploaded) but after 2 or 3 second it says (error).
I try to edit my code and my config file but i cant figure out where is the mistake.
I downloaded ajax control toolkit via nuget in vs 2015 community edition.
This is my aspx code
div class="form-group" >
<span class="label label-default"><asp:Literal id="AboutUsLabel" runat="server" />Hakkımızda Türkçe</span><br/>
<%-- <asp:TextBox id="AboutUsText" CssClass="form-control" style="resize:vertical" TextMode="multiline" Columns="50" Rows="20" runat="server" />--%>
<asp:TextBox id="AboutUs" CssClass="form-control" style="resize:vertical" TextMode="multiline" Columns="50" Rows="20" runat="server" />
<ajaxToolkit:HtmlEditorExtender ID="AboutUsEditor" runat="server"
OnImageUploadComplete="AboutUsEditor_ImageUploadComplete"
TargetControlID="AboutUs" EnableSanitization="false">
<Toolbar>
<ajaxToolkit:Undo />
<ajaxToolkit:Redo />
<ajaxToolkit:Bold />
<ajaxToolkit:Italic />
<ajaxToolkit:Underline />
<ajaxToolkit:StrikeThrough />
<ajaxToolkit:Subscript />
<ajaxToolkit:Superscript />
<ajaxToolkit:JustifyLeft />
<ajaxToolkit:JustifyCenter />
<ajaxToolkit:JustifyRight />
<ajaxToolkit:JustifyFull />
<ajaxToolkit:InsertOrderedList />
<ajaxToolkit:InsertUnorderedList />
<ajaxToolkit:CreateLink />
<ajaxToolkit:UnLink />
<ajaxToolkit:RemoveFormat />
<ajaxToolkit:SelectAll />
<ajaxToolkit:UnSelect />
<ajaxToolkit:Delete />
<ajaxToolkit:Cut />
<ajaxToolkit:Copy />
<ajaxToolkit:Paste />
<ajaxToolkit:BackgroundColorSelector />
<ajaxToolkit:ForeColorSelector />
<ajaxToolkit:FontNameSelector />
<ajaxToolkit:FontSizeSelector />
<ajaxToolkit:Indent />
<ajaxToolkit:Outdent />
<ajaxToolkit:InsertHorizontalRule />
<ajaxToolkit:HorizontalSeparator />
<ajaxToolkit:InsertImage />
</Toolbar>
</ajaxToolkit:HtmlEditorExtender>
</div>
.cs file code
protected void AboutUsEditor_ImageUploadComplete(object sender, AjaxFileUploadEventArgs e)
{
string fullPath = "/assets/images/homepage_image/" + e.FileName;
AboutUsEditor.AjaxFileUpload.SaveAs(MapPath("/assets/images/homepage_image/" + e.FileName));
e.PostedUrl = Page.ResolveUrl(fullPath);
}
and this code inside in system.web
<httpHandlers>
<add verb="*" path="AjaxFileUploadHandler.axd" type="AjaxControlToolkit.AjaxFileUploadHandler, AjaxControlToolkit"/>
</httpHandlers>
also this code inside in system.webserver
<validation validateIntegratedModeConfiguration="false"/>
<handlers>
<add name="AjaxFileUploadHandler" verb="*" path="AjaxFileUploadHandler.axd" type="AjaxControlToolkit.AjaxFileUploadHandler, AjaxControlToolkit"/>
</handlers>
<security>
<requestFiltering>
<requestLimits maxAllowedContentLength="4294967295"/>
</requestFiltering>
</security>