diff --git a/core/modules/file/src/Plugin/Field/FieldType/FileItem.php b/core/modules/file/src/Plugin/Field/FieldType/FileItem.php
index ba50903..0b59356 100644
--- a/core/modules/file/src/Plugin/Field/FieldType/FileItem.php
+++ b/core/modules/file/src/Plugin/Field/FieldType/FileItem.php
@@ -249,8 +249,9 @@ public static function validateExtensions($element, FormStateInterface $form_sta
    * fieldSettingsForm().
    */
   public static function validateMaxFilesize($element, FormStateInterface $form_state) {
-    if (!empty($element['#value']) && !is_numeric(Bytes::toInt($element['#value']))) {
-      $form_state->setError($element, t('The "@name" option must contain a valid value. You may either leave the text field empty or enter a string like "512" (bytes), "80 KB" (kilobytes) or "50 MB" (megabytes).', array('@name' => $element['title'])));
+    $max_size = Bytes::toInt($element['#value']);
+    if (!empty($element['#value']) && $max_size == 0) {
+      $form_state->setError($element, t('The "@name" option must contain a valid value. You may either leave the text field empty or enter a string like "512" (bytes), "80 KB" (kilobytes) or "50 MB" (megabytes).', array('@name' => $element['#title'])));
     }
   }
 
diff --git a/core/modules/file/src/Tests/FileFieldDisplayTest.php b/core/modules/file/src/Tests/FileFieldDisplayTest.php
index aa4923c..19716ba 100644
--- a/core/modules/file/src/Tests/FileFieldDisplayTest.php
+++ b/core/modules/file/src/Tests/FileFieldDisplayTest.php
@@ -170,4 +170,35 @@ function testDescToggle() {
     $this->assertText(t('The description may be used as the label of the link to the file.'));
   }
 
+  /**
+   * Test field settings form.
+   */
+  function testFileFieldSettings() {
+    $field_name = strtolower($this->randomMachineName());
+    $type_name = 'article';
+    $field_storage_settings = [
+      'display_field' => '1',
+      'display_default' => '0',
+      'cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED,
+    ];
+    $field_settings = [];
+    $widget_settings = [];
+    $field = $this->createFileField($field_name, 'node', $type_name, $field_storage_settings, $field_settings, $widget_settings);
+    $edit[0]['settings[max_filesize]'] = '@@';
+    $edit[1]['settings[max_filesize]'] = '0 MB';
+    $edit[2]['settings[max_filesize]'] = '-1 KB';
+    foreach($edit as $i){
+      $this->drupalPostForm('admin/structure/types/manage/article/fields/node.article.' . $field_name, $i, 'Save settings');
+      $this->assertText('The "Maximum upload size" option must contain a valid value. You may either leave the text field empty or enter a string like "512" (bytes), "80 KB" (kilobytes) or "50 MB" (megabytes).', 'Error message displayed for invalid max upload size');
+    }
+    $edit_1[0]['settings[max_filesize]'] = '10';
+    $edit_1[1]['settings[max_filesize]'] = '10 KB';
+    $edit_1[2]['settings[max_filesize]'] = '10 MB';
+    foreach($edit_1 as $i){
+      $this->drupalPostForm('admin/structure/types/manage/article/fields/node.article.' . $field_name, $i, 'Save settings');
+      $this->assertText('Saved ' . $field_name . ' configuration.');
+      $this->assertNoText('The "Maximum upload size" option must contain a valid value. You may either leave the text field empty or enter a string like "512" (bytes), "80 KB" (kilobytes) or "50 MB" (megabytes).', 'Error message not displayed for valid max upload size');
+    }
+  }
+
 }