Customizer: Remove file type check from config import

Fixes #14978.

docs/assets/js/src/customizer.js

@@ -361,20 +361,15 @@ window.onload = function () { // wait for load in a dumb way because B-0
 
     var file = (e.originalEvent.hasOwnProperty('dataTransfer')) ? e.originalEvent.dataTransfer.files[0] : e.originalEvent.target.files[0]
 
-    if (!file.type.match('application/json')) {
-      return showAlert('danger', '<strong>Ruh roh.</strong> We can only read <code>.json</code> files. Please try again.', importDropTarget)
-    }
-
     var reader = new FileReader()
 
-    reader.onload = (function () {
-      return function (e) {
+    reader.onload = function (e) {
       var text = e.target.result
 
       try {
         var json = JSON.parse(text)
 
-          if (typeof json != 'object') {
+        if (!$.isPlainObject(json)) {
           throw new Error('JSON data from config file is not an object.')
         }
 
@@ -384,9 +379,8 @@ window.onload = function () { // wait for load in a dumb way because B-0
         return showAlert('danger', '<strong>Shucks.</strong> We can only read valid <code>.json</code> files. Please try again.', importDropTarget)
       }
     }
-    })(file)
 
-    reader.readAsText(file)
+    reader.readAsText(file, 'utf-8')
   }
 
   function handleConfigDragOver(e) {