diff -u b/core/modules/file/file.es6.js b/core/modules/file/file.es6.js
--- b/core/modules/file/file.es6.js
+++ b/core/modules/file/file.es6.js
@@ -19,53 +19,23 @@
    *   Detaches validation for file extensions.
    */
   Drupal.behaviors.fileValidateAutoAttach = {
-<<<<<<< HEAD
-    attach(context, settings) {
+    attach: function (context) {
       const $context = $(context);
       let elements;
 
-      function initFileValidation(selector) {
-        $context.find(selector)
-          .once('fileValidate')
-          .on('change.fileValidate', { extensions: elements[selector] }, Drupal.file.validateExtension);
-      }
-
-      if (settings.file && settings.file.elements) {
-        elements = settings.file.elements;
-        Object.keys(elements).forEach(initFileValidation);
-      }
-    },
-    detach(context, settings, trigger) {
-      const $context = $(context);
-      let elements;
-=======
-    attach: function (context) {
-      var $context = $(context);
-      var elements;
-
       $context.find('[data-drupal-validate-extensions]')
         .once('fileValidate')
         .on('change.fileValidate', Drupal.file.validateExtension);
     },
-    detach: function (context, settings, trigger) {
-      var $context = $(context);
->>>>>>> Applying patch from issue 2235977 comment 114
+    detach(context, settings, trigger) {
+      const $context = $(context);
 
       if (trigger === 'unload') {
         $context.find('[data-drupal-validate-extensions]')
           .removeOnce('fileValidate')
           .off('change.fileValidate', Drupal.file.validateExtension);
       }
-<<<<<<< HEAD
-
-      if (trigger === 'unload' && settings.file && settings.file.elements) {
-        elements = settings.file.elements;
-        Object.keys(elements).forEach(removeFileValidation);
-      }
-    },
-=======
     }
->>>>>>> Applying patch from issue 2235977 comment 114
   };
 
   /**
@@ -152,13 +122,6 @@
       $('.file-upload-js-error').remove();
 
       // Add client side validation for the input[type=file].
-<<<<<<< HEAD
-      const extensionPattern = event.data.extensions.replace(/,\s*/g, '|');
-      if (extensionPattern.length > 1 && this.value.length > 0) {
-        const acceptableMatch = new RegExp(`\\.(${extensionPattern})$`, 'gi');
-        if (!acceptableMatch.test(this.value)) {
-          const error = Drupal.t('The selected file %filename cannot be uploaded. Only files with the following extensions are allowed: %extensions.', {
-=======
       var validExtensions = JSON.parse(event.target.getAttribute('data-drupal-validate-extensions'));
       if (validExtensions.length > 1 && event.target.value.length > 0) {
         var acceptableMatch = new RegExp('\\.(' + validExtensions.join('|') + ')$', 'i');
@@ -170,7 +133,6 @@
         for (var i = 0; i < testSubjects.length; i++) {
           var testSubject = testSubjects[i];
           if (!acceptableMatch.test(testSubject.name)) {
->>>>>>> Applying patch from issue 2235977 comment 114
             // According to the specifications of HTML5, a file upload control
             // should not reveal the real local path to the file that a user
             // has selected. Some web browsers implement this restriction by
@@ -178,18 +140,11 @@
             // confusion by leaving the user thinking perhaps Drupal could not
             // find the file because it messed up the file path. To avoid this
             // confusion, therefore, we strip out the bogus fakepath string.
-<<<<<<< HEAD
-            '%filename': this.value.replace('C:\\fakepath\\', ''),
-            '%extensions': extensionPattern.replace(/\|/g, ', '),
-          });
-          $(this).closest('div.js-form-managed-file').prepend(`<div class="messages messages--error file-upload-js-error" aria-live="polite">${error}</div>`);
-=======
             invalidFiles.push(testSubject.name.replace('C:\\fakepath\\', ''));
           }
         }
         if (invalidFiles.length) {
           $(this).closest('div.js-form-managed-file').prepend(Drupal.theme('fileValidationError', invalidFiles, validExtensions));
->>>>>>> Applying patch from issue 2235977 comment 114
           this.value = '';
           // Cancel all other change event handlers.
           event.stopImmediatePropagation();
@@ -289,9 +244,6 @@
       window.open(this.href, 'filePreview', 'toolbar=0,scrollbars=1,location=1,statusbar=1,menubar=0,resizable=1,width=500,height=550');
     },
   };
-<<<<<<< HEAD
-}(jQuery, Drupal));
-=======
 
   /**
    * Error message of file validation.
@@ -326,2 +278 @@
-})(jQuery, Drupal);
->>>>>>> Applying patch from issue 2235977 comment 114
+}(jQuery, Drupal));
diff -u b/core/modules/file/file.js b/core/modules/file/file.js
--- b/core/modules/file/file.js
+++ b/core/modules/file/file.js
@@ -15,14 +15,6 @@
     },
     detach: function detach(context, settings, trigger) {
       var $context = $(context);
-<<<<<<< HEAD
-      var elements = void 0;
-
-      function removeFileValidation(selector) {
-        $context.find(selector).removeOnce('fileValidate').off('change.fileValidate', Drupal.file.validateExtension);
-      }
-=======
->>>>>>> Applying patch from issue 2235977 comment 114
 
       if (trigger === 'unload') {
         $context.find('[data-drupal-validate-extensions]').removeOnce('fileValidate').off('change.fileValidate', Drupal.file.validateExtension);
@@ -157 +149 @@
-})(jQuery, Drupal);
\ No newline at end of file
+})(jQuery, Drupal);