diff -u b/core/modules/file/file.post_update.php b/core/modules/file/file.post_update.php
--- b/core/modules/file/file.post_update.php
+++ b/core/modules/file/file.post_update.php
@@ -10,8 +10,7 @@
 use Drupal\file\Plugin\Field\FieldType\FileItem;
 
 /**
- * Adds txt to allowed extensions for all fields that allow uploads of insecure
- * files.
+ * Add txt to allowed extensions for all fields that allow uploads of insecure files.
  */
 function file_post_update_update_allowed_file_extensions_if_insecure(&$sandbox = NULL) {
   if (\Drupal::config('system.file')->get('allow_insecure_uploads')) {
@@ -30,7 +29,7 @@
       foreach ($allowed_extensions as $extension) {
         if (preg_match(FILE_INSECURE_EXTENSION_REGEX, 'test.' . $extension)) {
           // Add the txt extension to the list of allowed extensions if an
-          // insecure (one|extension) is allowed.
+          // insecure extension is allowed.
           $allowed_extensions_string .= ' txt';
           $field->setSetting('file_extensions', $allowed_extensions_string);
           return TRUE;
diff -u b/core/modules/file/src/Plugin/Field/FieldType/FileItem.php b/core/modules/file/src/Plugin/Field/FieldType/FileItem.php
--- b/core/modules/file/src/Plugin/Field/FieldType/FileItem.php
+++ b/core/modules/file/src/Plugin/Field/FieldType/FileItem.php
@@ -236,17 +236,11 @@
 
       // If insecure uploads are not allowed then error if txt is not an allowed
       // extension.
-      if (!in_array('txt', $extension_array, TRUE) && !\Drupal::config('system.file')->get('allow_insecure_uploads')) {
-        $unique_extensions = [];
-        // Filter out definitions that are not intended to be placed by the UI.
-        $unique_extensions = array_filter($extension_array, function (string $extension) {
-          if (!in_array($extension, $unique_extensions) && preg_match(FILE_INSECURE_EXTENSION_REGEX, 'test.' . $extension)) {
-            return $extension;
+      if (!in_array('txt', $extension_array) && !\Drupal::config('system.file')->get('allow_insecure_uploads')) {
+        foreach ($extension_array as $extension) {
+          if (preg_match(FILE_INSECURE_EXTENSION_REGEX, 'test.' . $extension)) {
+            $form_state->setError($element, t('Add %txt_extension to the list of allowed extensions to securely upload files with a %extension extension. The %txt_extension extension will then be added automatically.', ['%extension' => $extension, '%txt_extension' => 'txt']));
           }
-        });
-        if (!empty($unique_extensions)) {
-          $unique_extensions = implode(' or ', $unique_extensions);
-          $form_state->setError($element, t('Add %txt_extension to the list of allowed extensions to securely upload files with a %extension extension. The %txt_extension extension will then be added automatically.', ['%extension' => $unique_extensions, '%txt_extension' => 'txt']));
         }
       }
     }