diff -u b/core/lib/Drupal/Core/Session/SessionManager.php b/core/lib/Drupal/Core/Session/SessionManager.php
--- b/core/lib/Drupal/Core/Session/SessionManager.php
+++ b/core/lib/Drupal/Core/Session/SessionManager.php
@@ -284,6 +284,11 @@
     $this->writeSafeHandler = $handler;
   }
 
+  /**
+   * Returns the built-in PHP session handler.
+   *
+   * @return string
+   */
   public function getPHPSessionHandler() {
     return $this->PHPSessionHandler;
   }
diff -u b/core/modules/file/file.es6.js b/core/modules/file/file.es6.js
--- b/core/modules/file/file.es6.js
+++ b/core/modules/file/file.es6.js
@@ -222,11 +222,14 @@
       if ($progressId.length) {
         const originalName = $progressId.attr('name');
 
-        // Replace the name with the last string in the name that does not
-        // contain square brackets.
+        // Replace the name with the required identifier to ensure that this
+        // element sends its upload identifier. The upload identifier is the
+        // last string in the name that does not contain square brackets.
         $progressId.attr('name', originalName.match(/[^\[\]]+(?!.*[^\[\]]+)/)[0]);
 
-        // Restore the original name after the upload begins.
+        // Restore the original name after the upload begins to prevent this
+        // element from sending its upload identifier with another element’s
+        // upload.
         setTimeout(() => {
           $progressId.attr('name', originalName);
         }, 1000);
diff -u b/core/modules/file/src/Controller/FileWidgetAjaxController.php b/core/modules/file/src/Controller/FileWidgetAjaxController.php
--- b/core/modules/file/src/Controller/FileWidgetAjaxController.php
+++ b/core/modules/file/src/Controller/FileWidgetAjaxController.php
@@ -77,25 +77,23 @@
       // one generated by Drupal so PHP can recognize the browser’s cookie
       // before Drupal starts.
 
+      // Stop the Drupal session temporarily. Save the session handler and data
+      // so we can resume it later.
       $save_handler = session_module_name();
       $session = $_SESSION;
-
-      // Stop the current session without saving any data to keep your original data
-      // in your path.
       session_abort();
 
-      // Get upload status from the built-in PHP session data.
+      // Get upload status from the PHP session.
       $status = [];
       session_module_name($this->PHPSessionHandler);
-      // Fail silently if built-in PHP session handler generates an error.
+      // Fail silently if the PHP session handler generates an error.
       @session_start();
       $prefix = ini_get('session.upload_progress.prefix');
       if (isset($_SESSION[$prefix . $key])) {
         $status = $_SESSION[$prefix . $key];
       }
 
-      // Close the built-in PHP session without saving and restore the current
-      // session.
+      // Stop the PHP session and resume the Drupal session.
       session_abort();
       session_module_name($save_handler);
       session_start();