diff --git a/core/lib/Drupal/Core/Field/FieldFilteredString.php b/core/lib/Drupal/Core/Field/FieldFilteredString.php
index c3fca4d..d41c366 100644
--- a/core/lib/Drupal/Core/Field/FieldFilteredString.php
+++ b/core/lib/Drupal/Core/Field/FieldFilteredString.php
@@ -78,8 +78,16 @@ public static function displayAllowedTags() {
   }
 
   /**
-   * @param $option
-   * @return $this|static
+   * Transforms the string given an option.
+   *
+   * @param string $option
+   *   One of the following:
+   *   - upper: makes the string uppercase.
+   *   - lower: makes the string lowercase.
+   *   - ucfirst: makes the string's first character uppercase.
+   *   - ucwords: uppercase the first character of each word in the string.
+   *
+   * @return static
    *   If the case transformation does not affect the string the same object is
    *   returned. If it does, then a new object is returned, preserving
    *   immutability.
diff --git a/core/modules/file/file.field.inc b/core/modules/file/file.field.inc
index 5c9bd68..a80a4bd 100644
--- a/core/modules/file/file.field.inc
+++ b/core/modules/file/file.field.inc
@@ -10,7 +10,6 @@
 use Drupal\Core\Field\FieldFilteredString;
 use Drupal\Core\Render\Element;
 
-
 /**
  * Returns HTML for an individual file upload widget.
  *
diff --git a/core/tests/Drupal/Tests/Core/Field/FieldFilteredStringTest.php b/core/tests/Drupal/Tests/Core/Field/FieldFilteredStringTest.php
new file mode 100644
index 0000000..9546b08
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Field/FieldFilteredStringTest.php
@@ -0,0 +1,101 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\Core\Field\FieldFilteredStringTest.
+ */
+
+namespace Drupal\Tests\Core\Field;
+
+use Drupal\Tests\UnitTestCase;
+use Drupal\Core\Field\FieldFilteredString;
+use Drupal\Component\Utility\SafeStringInterface;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Field\FieldFilteredString
+ * @group Field
+ */
+class FieldFilteredStringTest extends UnitTestCase {
+
+  /**
+   * @covers ::create
+   * @dataProvider providerTestCreate
+   */
+  public function testCreate($string, $expected, $instance_of_check = FALSE) {
+    $filtered_string = FieldFilteredString::create($string);
+
+    if ($instance_of_check) {
+      $this->assertInstanceOf(FieldFilteredString::class, $filtered_string);
+    }
+    $this->assertSame($expected, (string) $filtered_string);
+  }
+
+  /**
+   * Provides data for testCreate().
+   */
+  public function providerTestCreate() {
+    $data = [];
+    $data[] = [
+      '', '',
+    ];
+    $data[] = [
+      'teststring', 'teststring', TRUE
+    ];
+
+    $safe_string = $this->prophesize(SafeStringInterface::class);
+    $safe_string->__toString()->willReturn('teststring');
+
+    $data[] = [
+      $safe_string->reveal(), 'teststring', TRUE
+    ];
+    return $data;
+  }
+
+  /**
+   * @covers: ::allowedTags
+   */
+  public function testAllowedTags() {
+    $expected = ['a', 'b', 'big', 'code', 'del', 'em', 'i', 'ins',  'pre', 'q', 'small', 'span', 'strong', 'sub', 'sup', 'tt', 'ol', 'ul', 'li', 'p', 'br', 'img'];
+
+    $this->assertSame($expected, FieldFilteredString::allowedTags());
+  }
+
+  /**
+   * @covers: ::displayAllowedTags
+   */
+  public function testdisplayAllowedTags() {
+    $expected = '<a> <b> <big> <code> <del> <em> <i> <ins> <pre> <q> <small> <span> <strong> <sub> <sup> <tt> <ol> <ul> <li> <p> <br> <img>';
+
+    $this->assertSame($expected, FieldFilteredString::displayAllowedTags());
+  }
+
+  /**
+   * @covers ::caseTransform
+   * @dataProvider providertestCaseTransform
+   */
+  public function testCaseTransform($option, $test_string, $expected) {
+    $filtered_string = FieldFilteredString::create($test_string);
+    $this->assertEquals($expected, $filtered_string->caseTransform($option));
+  }
+
+  /**
+   * Provides data for testCaseTransform().
+   */
+  public function providertestCaseTransform() {
+    $data = [];
+    $data[] = [
+      'upper', 'teststring', 'TESTSTRING'
+    ];
+    $data[] = [
+      'lower', 'TESTSTRING', 'teststring'
+    ];
+    $data[] = [
+      'ucfirst', 'teststring', 'Teststring'
+    ];
+    $data[] = [
+      'ucwords', 'teststring and teststring', 'Teststring And Teststring'
+    ];
+    return $data;
+  }
+
+}