ASP.NET

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim a As New DataSet1.TasksDataTable()
        Dim adp As New DataSet1TableAdapters.TasksTableAdapter
        adp.Fill(a)
        GridView1.DataSource = a
        GridView1.DataBind()
        GridView1.Visible = True
    End Sub
 

$("#btnUploadFile").click(function () {
	    if (files.length <= 0) {
	        alert("Debe seleccionar algún fichero para subirlo.");
	    } else {
	        var expID = $("#ContentPlaceHolder1_hfExpId").val();
	        var formData = new FormData();
	        for (var i = 0; i < files.length; i++) {
	            alert(files[i].name);
	            formData.append('file', files[i]);
	        }
	        var xhr = new XMLHttpRequest();
	        xhr.open('POST', "FileHandler.ashx", true);
	        xhr.overrideMimeType('text/plain; charset=x-user-defined-binary');
	        xhr.setRequestHeader("ExpedienteID", expID);
	        xhr.onload = function () {
	            if (xhr.status === 200) {
	                RefreshFilePanel();
	            } else {
	                console.log('Something went terribly wrong...');
	            }
	        };
	        xhr.send(formData);
	    };

Public Sub ProcessRequest(ByVal context As HttpContext) Implements IHttpHandler.ProcessRequest
        Dim documentBytes As Byte()
        Dim lExpId As String = context.Request.Headers("ExpedienteID")
        Dim fLenght As Integer
        
        If (context.Request.Files.Count > 0) Then
            Dim files As HttpFileCollection = context.Request.Files
 
            For i = 0 To files.Count - 1
                fLenght = files(i).ContentLength
                documentBytes = New Byte(fLenght - 1) {}
                context.Request.InputStream.Read(documentBytes, 0, fLenght)
'This save byte() image on SQL Database:
                UploadDocumentBy_ExpID(documentBytes, files(i).FileName, "Description" & lExpId.ToString, lExpId)
            Next
        End If
    End Sub

<asp:GridView ID="grdDocumentoByExp" runat="server" AutoGenerateColumns="false" Width="250px"
 DataSourceID="dtsDocumentByExpId">
 <Columns>
  <asp:BoundField DataField="Archivo" HeaderText="Archivo" />
  <asp:BoundField DataField="docId" HeaderText="docId" />
  <asp:BoundField DataField="Documento" HeaderText="Documento" Visible="false" />
    <asp:TemplateField HeaderText="Preview">
       <ItemTemplate>
         <asp:Image 
         ID="imgThumb" 
         runat="server"
         ImageUrl='<%# GetImage(Eval("Documento")) %>'
        />
       </ItemTemplate>
    </asp:TemplateField>                                                                
 </Columns>
</asp:GridView>

    Public Function GetImage(image As Object) As String
        Return "data:image/gif;base64," & Convert.ToBase64String(DirectCast(image, [Byte]()))
    End Function

"Value does not fall within the expected range".

[NullReferenceException: Object reference not set to an instance of an object.]
   System.Web.Mvc.Controller.PossiblyLoadTempData() +52
   System.Web.Mvc.Controller.BeginExecuteCore(AsyncCallback callback, Object state) +92
   System.Web.Mvc.Async.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +161
   System.Web.Mvc.Async.AsyncResultWrapper.Begin(AsyncCallback callback, Object state, BeginInvokeDelegate beginDelegate, EndInvokeDelegate`1 endDelegate, Object tag, Int32 timeout) +92
   System.Web.Mvc.Async.AsyncResultWrapper.Begin(AsyncCallback callback, Object state, BeginInvokeDelegate beginDelegate, EndInvokeDelegate endDelegate, Object tag, Int32 timeout) +97
   System.Web.Mvc.Async.AsyncResultWrapper.Begin(AsyncCallback callback, Object state, BeginInvokeDelegate beginDelegate, EndInvokeDelegate endDelegate, Object tag) +53
   System.Web.Mvc.Controller.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +417
   System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +49
   System.Web.Mvc.<>c__DisplayClass8.<BeginProcessRequest>b__2(AsyncCallback asyncCallback, Object asyncState) +127
   System.Web.Mvc.Async.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +161
   System.Web.Mvc.Async.AsyncResultWrapper.Begin(AsyncCallback callback, Object state, BeginInvokeDelegate beginDelegate, EndInvokeDelegate`1 endDelegate, Object tag, Int32 timeout) +92
   System.Web.Mvc.Async.AsyncResultWrapper.Begin(AsyncCallback callback, Object state, BeginInvokeDelegate beginDelegate, EndInvokeDelegate endDelegate, Object tag, Int32 timeout) +97
   System.Web.Mvc.Async.AsyncResultWrapper.Begin(AsyncCallback callback, Object state, BeginInvokeDelegate beginDelegate, EndInvokeDelegate endDelegate, Object tag) +53
   System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) +373
   System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) +88
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) +50
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +301
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155