sponsored.js: Show error when trying to add files which are too large

r2/r2/public/static/js/sponsored.js

@@ -501,6 +501,23 @@ var exports = r.sponsored = {
         this.inventory = {}
         this.campaignListColumns = $('.existing-campaigns thead th').length
         $("input[name='media_url_type']").on("change", this.mediaInputChange)
+
+        $('input[type=file]').on('change', this._checkFilesize);
+    },
+
+    _checkFilesize: function(e) {
+      var el = e.target;
+      var $el = $(el);
+      var files = el.files;
+
+      if (files && files[0].size > 1024 * 500) {
+        $el
+          .wrap('<form>')
+          .closest('form')
+          .get(0).reset();
+        $el.unwrap();
+        alert(r._('too big. keep it under 500 KiB'));
+      }
     },
 
     setup: function(inventory_by_sr, priceDict, isEmpty, userIsSponsor) {