<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Test</title>
<link rel="stylesheet" href="http://cdn.leafletjs.com/leaflet-0.7.3/leaflet.css"/>
<script src="http://cdn.leafletjs.com/leaflet-0.7.3/leaflet.js"></script>
<script src="http://code.jquery.com/jquery-2.1.1.min.js"></script>
<script src="jssrc="https://cdnjs.cloudflare.com/ajax/libs/leaflet-ajax/2.1.0/leaflet.ajax.min.js"></script>
<style>
#map1 { height: 500px; width: 550px }
</style>
</head>
<body>
<div id="map1">
</div>
<script>
var mymap = L.map('map1').setView([43.370917,-80.983042], 13);
L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token=pk.eyJ1IjoibWFwYm94IiwiYSI6ImNpejY4NXVycTA2emYycXBndHRqcmZ3N3gifQ.rJcFIG214AriISLbB6B5aw', {
maxZoom: 18,
attribution: 'Map data © <a href="https://www.openstreetmap.org/">OpenStreetMap</a> contributors, ' +
'<a href="https://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, ' +
'Imagery © <a href="https://www.mapbox.com/">Mapbox</a>',
id: 'mapbox.streets'
}).addTo(mymap);
var geojson = new L.GeoJSON.AJAX("StratfordBoundary.geojson");
geojson.on('data:loaded', function(){
geojson.addTo(mymap);
});
</script>
</body>
</html>
However, the .html file above now works fine for me with my own test .geojson. Don't miss that I also changedjust drew a simple polygon in QGIS and saved it as StratfordBoundary.geojson (EPSG:4326) in the path forsame folder as the plugin.html file.