Remove the test for mime-type text/plain as this seems to be based on suffix only.

chrome/browser/resources/options/chromeos/internet_options.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 cr.define('options', function() {
   var OptionsPage = options.OptionsPage;
   const ArrayDataModel = cr.ui.ArrayDataModel;
 
   /////////////////////////////////////////////////////////////////////////////
   // InternetOptions class:
@@ skipping 72 matching lines @@
         chrome.send('disableCellular', []);
       });
       $('change-proxy-button').addEventListener('click', function(event) {
         OptionsPage.closeOverlay();
         OptionsPage.showPageByName('proxy', false);
         chrome.send('coreOptionsUserMetricsAction',
             ['Options_ShowProxySettings']);
       });
       $('upload-network-settings').addEventListener('change', function(event) {
         var file = event.target.files[0];
-        if (!file.type.match('text/plain')) {
-          InternetOptions.invalidNetworkSettings();
-          return;
-        }
-
         var reader = new FileReader();
         reader.onloadend = function(e) {
           chrome.send('importNetworkSettings', [this.result]);
         };
         reader.readAsText(file);
       }, false);
       $('buyplanDetails').addEventListener('click', function(event) {
         chrome.send('buyDataPlan', []);
         OptionsPage.closeOverlay();
       });
@@ skipping 559 matching lines @@
 
   InternetOptions.invalidNetworkSettings = function () {
     alert(localStrings.getString('invalidNetworkSettings'));
   };
 
   // Export
   return {
     InternetOptions: InternetOptions
   };
 });