Problem with using search Filter - how to set the Filter path correctly?

Hello everybody,

my search filter doesn't work. I get this error in the console when it tries to create the Filter:

GET http://localhost:63342/MessageSender/firstKey 404 (Not Found)

That's weird, because the location of my .json file is http://localhost:63342/MessageSender/payloads/payloads.json

I tried to change the path (first argument) when creating the filter from firstkey into this String:

"payloads/payloads.json#/data/firstKey"

but then I got no error in the console, and the search filter still didn't work. How do I set the Filter path correctly so the search element works?

My controller:

sap.ui.define([
   'jquery.sap.global',
   'sap/ui/core/mvc/Controller',
   'sap/ui/core/Fragment',
   'sap/ui/model/Filter',
   'sap/ui/model/json/JSONModel',
], function(jQuery, Controller, JSONModel, Filter, Fragment) {
   "use strict";

   var SenderController = Controller.extend("messagesender.Sender", {
   onInit: function() {
        this._payloadsModelJSON = new sap.ui.model.json.JSONModel();
        this._oPayloadSelectDialog = sap.ui.xmlfragment("./messagesender.PayloadSelectDialog", this);
        this._oPayloadsModelJSON.loadData("payloads/payloads.json", '', false); //false for synchronous operation
        this._oPayloadSelectDialog.setModel(this._oPayloadsModelJSON);
   }

   handlePayloadSelectDialogSearch: function(oEvent) {
        var sValue = oEvent.getParameter("value");
        var oFilter = new Filter("firstKey", sap.ui.model.FilterOperator.Contains, sValue);
        var oBinding = oEvent.getSource().getBinding("items");
        oBinding.filter([oFilter]);
   }
}

My fragment:

<core:FragmentDefinition xmlns="sap.m" xmlns:core="sap.ui.core">
    <SelectDialog
            noDataText="No Payloads Found"
            title="Select Saved Payload"
            search="handlePayloadSelectDialogSearch"
            confirm="handlePayloadSelectDialogClose"
            close="handlePayloadSelectDialogClose"
            items="{/data}" >
        <CustomListItem type="Active" >
            <HBox
                    height="64px"
                    width="480px"
                    alignItems="Center"
                    class="sapUiSmallMarginBegin"
                    justifyContent="SpaceBetween">
                <VBox id="VBoxID">
                    <Label text="{firstKey}" class="sapMSLITitle" />
                    <Label text="{secondKey}" visible="false" />
                    <Label text="{thirdKey}" class="sapMSLIDescription"/>
                </VBox>
                <Button icon="sap-icon://display" class="sapUiSmallMarginEnd" press="handlePreviewPayload"/>
            </HBox>
        </CustomListItem>
    </SelectDialog>
</core:FragmentDefinition>

And payloads/payloads.json file:

{"data": [{"firstKey":"request.xml"},{"firstKey":"request_live_nemo.xml"},{"firstKey":"request_migration.xml"},{"firstKey":"request_myriad.xml"},{"firstKey":"request_nemo.xml"},{"firstKey":"request_nemo_no_myriad.xml"},{"firstKey":"request_nemo_no_service_number.xml"},{"firstKey":"request_no_myriad.xml"}
]}