diff --git a/core/modules/file/src/Plugin/Field/FieldType/FileItemMaxFileSizeValidator.php b/lib/Drupal/Component/Utility/BytesValidator.php
similarity index 73%
rename from core/modules/file/src/Plugin/Field/FieldType/FileItemMaxFileSizeValidator.php
rename to lib/Drupal/Component/Utility/BytesValidator.php
index 713cf743af..1b8a1826e4 100644
--- a/core/modules/file/src/Plugin/Field/FieldType/FileItemMaxFileSizeValidator.php
+++ b/lib/Drupal/Component/Utility/BytesValidator.php
@@ -1,15 +1,11 @@
 <?php

-namespace Drupal\file\Plugin\Field\FieldType;
-
-use Drupal\Component\Utility\Bytes;
-use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\StringTranslation\TranslatableMarkup;
+namespace Drupal\Component\Utility;

 /**
  * Provides helper methods for max file size value validation.
  */
-class FileItemMaxFileSizeValidator {
+class BytesValidator {

   /**
    * Holds a string representing a byte size to be validated.
@@ -26,34 +22,33 @@ class FileItemMaxFileSizeValidator {
    * Ensures that a size has been entered and that it can be parsed by
    * \Drupal\Component\Utility\Bytes::toInt().
    *
-   * This function is assigned as an #element_validate callback in
-   * fieldSettingsForm().
+   * @param string $byte_size_string
+   *   A user entered string representing a byte size to be validated.
    *
-   * @param array $element
-   *   A field element array.
-   * @param \Drupal\Core\Form\FormStateInterface $form_state
-   *   The current state of the form.
+   * @return bool
+   *   TRUE if meaningful byte size string, FALSE if not.
    */
-  public static function validateMaxFilesize(array $element, FormStateInterface $form_state) {
+  public static function validateByteSizeString($byte_size_string) {

     // Spaces are irrelevant to us. Bytes::toInt will process with or without
     // them. Make our lives easier by just doing away with them.
-    self::$size = self::stripWhitespaceFromString($element['#value']);
+    self::$size = self::stripWhitespaceFromString($byte_size_string);

     // Having removed whitespace, if our string is now empty then the user
     // either entered nothing or a string of whitespace. Accept both cases as
     // valid.
     if (empty(self::$size)) {
-      return;
+      return TRUE;
     }

     // Theoretically self::stringIsValidByteString() should perform strict
     // enough validation. Passing the string to Bytes::toInt() for added
     // precaution.
-    if (self::sizeIsValidByteString() || !self::toIntCanParseString()) {
-      $translatable_markup = new TranslatableMarkup('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).', ['@name' => $element['#title']]);
-      $form_state->setError($element, $translatable_markup);
+    if (!self::sizeIsValidByteString() || !self::toIntCanParseString()) {
+      return FALSE;
     }
+
+    return TRUE;
   }

   /**
@@ -63,7 +58,7 @@ class FileItemMaxFileSizeValidator {
    *   TRUE if size is a valid byte size string, FALSE if not.
    */
   private static function sizeIsValidByteString() {
-    return (
+    return !(
       self::sizeIsNotAlphaNumeric() ||
       self::sizeNotStartWithNumber() ||
       self::sizeHasNumbersFollowingAlphaChar()
diff --git a/core/modules/file/src/Plugin/Field/FieldType/FileItem.php b/core/modules/file/src/Plugin/Field/FieldType/FileItem.php
index c2400fe564..8f6849594a 100644
--- a/core/modules/file/src/Plugin/Field/FieldType/FileItem.php
+++ b/core/modules/file/src/Plugin/Field/FieldType/FileItem.php
@@ -2,6 +2,7 @@

 namespace Drupal\file\Plugin\Field\FieldType;

+use Drupal\Component\FileSystem\BytesValidator;
 use Drupal\Component\Utility\Bytes;
 use Drupal\Component\Render\PlainTextOutput;
 use Drupal\Component\Utility\Environment;
@@ -12,6 +13,7 @@ use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;
 use Drupal\Core\File\FileSystemInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\StreamWrapper\StreamWrapperInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
 use Drupal\Core\TypedData\DataDefinition;

 /**
@@ -181,7 +183,7 @@ class FileItem extends EntityReferenceItem {
       '#default_value' => $settings['max_filesize'],
       '#description' => t('Enter a value like "512" (bytes), "80 KB" (kilobytes) or "50 MB" (megabytes) in order to restrict the allowed file size. If left empty the file sizes will be limited only by PHP\'s maximum post and file upload sizes (current limit <strong>%limit</strong>).', ['%limit' => format_size(Environment::getUploadMaxSize())]),
       '#size' => 10,
-      '#element_validate' => [[FileItemMaxFileSizeValidator::class, 'validateMaxFilesize']],
+      '#element_validate' => [[get_class($this), 'validateExtensions']],
       '#weight' => 5,
     ];

@@ -237,6 +239,22 @@ class FileItem extends EntityReferenceItem {
     }
   }

+  /**
+   * Form API callback.
+   *
+   * Ensures that a valid size has been entered and that it can be parsed by
+   * \Drupal\Component\Utility\Bytes::toInt().
+   *
+   * This function is assigned as an #element_validate callback in
+   * fieldSettingsForm().
+   */
+  public static function validateMaxFilesize($element, FormStateInterface $form_state) {
+    if (BytesValidator::validateByteSizeString()) {
+      $translatable_markup = new TranslatableMarkup('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).', ['@name' => $element['#title']]);
+      $form_state->setError($element, $translatable_markup);
+    }
+  }
+
   /**
    * Determines the URI for a file field.
    *
diff --git a/core/modules/file/tests/src/Unit/BytesValidatorTest.php b/core/modules/file/tests/src/Unit/BytesValidatorTest.php
new file mode 100644
index 0000000000..f03b0c15a0
--- /dev/null
+++ b/core/modules/file/tests/src/Unit/BytesValidatorTest.php
@@ -0,0 +1,85 @@
+<?php
+
+namespace Drupal\Tests\file\Unit;
+
+use Drupal\Component\Utility\BytesValidator;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests FileItem's settings form's validation.
+ *
+ * @group file
+ */
+class BytesValidatorTest extends UnitTestCase {
+
+  /**
+   * Tests that a sane Max file size is entered.
+   *
+   * @dataProvider maxFileSizeInputsProvider
+   */
+  public function testValidateByteSizeString($input, $expected) {
+    $result = BytesValidator::validateByteSizeString($input);
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * Data provider for testValidateMaxfilesize().
+   *
+   * @return array[]
+   *   Array of inputs and error expectations.
+   */
+  public function maxFileSizeInputsProvider() {
+    $data = [
+      ['0 bytes', TRUE],
+      ['   ', TRUE],
+      ['', TRUE],
+      ['0 K', TRUE],
+      ['100bytes', TRUE],
+      ['10000', TRUE],
+      ['100 bytes', TRUE],
+      ['999989 bytes', TRUE],
+      ['999989bytes', TRUE],
+      ['2', TRUE],
+      ['3K', TRUE],
+      ['5MB', TRUE],
+      ['10G', TRUE],
+      ['6GiB', TRUE],
+      ['8bytes', TRUE],
+      ['9mbytes', TRUE],
+      // Should we allow these spaces?
+      ['1 0 0 0 0 m b y tes', TRUE],
+      [' 123456 ', TRUE],
+
+      ['123456.03 ', FALSE],
+      ['nonumbers', FALSE],
+      ['bread', FALSE],
+      ['bananas', FALSE],
+      ['1234b1', FALSE],
+      ['543xd1', FALSE],
+      ['-1', FALSE],
+      ['-975', FALSE],
+      [' word ', FALSE],
+      [' word', FALSE],
+      ['word ', FALSE],
+      [' words with spaces ', FALSE],
+      ['words with spaces ', FALSE],
+      [' words with spaces', FALSE],
+      [' 123456.03 ', FALSE],
+      ['123456.03 ', FALSE],
+      [' 123456.03', FALSE],
+      [' some words & stuff', FALSE],
+    ];
+
+    // Improve test output by using the input value as a label for the dataset.
+    foreach ($data as $index => $item) {
+      // If the index looks like an in eg "1000" "2" etc, php still detects
+      // it as a numeric index. Adding text to all the labels to force them
+      // to render.
+      $data["Sample input: '{$item[0]}'"] = $item;
+      unset($data[$index]);
+    }
+
+    return $data;
+  }
+
+}
diff --git a/core/modules/file/tests/src/Unit/FileItemFieldSettingsFormTest.php b/core/modules/file/tests/src/Unit/FileItemFieldSettingsFormTest.php
deleted file mode 100644
index 65de0d88dd..0000000000
--- a/core/modules/file/tests/src/Unit/FileItemFieldSettingsFormTest.php
+++ /dev/null
@@ -1,102 +0,0 @@
-<?php
-
-namespace Drupal\Tests\file\Unit;
-
-use Drupal\Core\Form\FormState;
-use Drupal\Core\StringTranslation\TranslatableMarkup;
-use Drupal\file\Plugin\Field\FieldType\FileItemMaxFileSizeValidator;
-use Drupal\Tests\UnitTestCase;
-use Prophecy\Argument;
-
-/**
- * Tests FileItem's settings form's validation.
- *
- * @group file
- */
-class FileItemFieldSettingsFormTest extends UnitTestCase {
-
-  /**
-   * Tests that a sane Max file size is entered.
-   *
-   * @dataProvider maxFileSizeInputsProvider
-   */
-  public function testValidateMaxfilesize($input, $should_error) {
-
-    $element = [
-      '#title' => 'Maximum upload size',
-      '#value' => $input,
-    ];
-
-    $form_state = $this->prophesize(FormState::class);
-    if ($should_error) {
-      $form_state->setError($element, Argument::type(TranslatableMarkup::class))
-        ->shouldBeCalled();
-    }
-    else {
-      $form_state->setError($element, Argument::type(TranslatableMarkup::class))
-        ->shouldNotBeCalled();
-    }
-    FileItemMaxFileSizeValidator::validateMaxFilesize($element, $form_state->reveal());
-
-  }
-
-  /**
-   * Data provider for testValidateMaxfilesize().
-   *
-   * @return array[]
-   *   Array of inputs and error expectations.
-   */
-  public function maxFileSizeInputsProvider() {
-    $data = [
-      ['0 bytes', FALSE],
-      ['123456.03 ', TRUE],
-      ['   ', FALSE],
-      ['', FALSE],
-      ['0 K', FALSE],
-      ['100bytes', FALSE],
-      ['10000', FALSE],
-      ['100 bytes', FALSE],
-      ['999989 bytes', FALSE],
-      ['999989bytes', FALSE],
-      ['2', FALSE],
-      ['3K', FALSE],
-      ['5MB', FALSE],
-      ['10G', FALSE],
-      ['6GiB', FALSE],
-      ['8bytes', FALSE],
-      ['9mbytes', FALSE],
-      // Should we allow these spaces?
-      ['1 0 0 0 0 m b y tes', FALSE],
-      ['nonumbers', TRUE],
-      ['bread', TRUE],
-      ['bananas', TRUE],
-      ['1234b1', TRUE],
-      ['543xd1', TRUE],
-      ['-1', TRUE],
-      ['-975', TRUE],
-      [' word ', TRUE],
-      [' word', TRUE],
-      ['word ', TRUE],
-      [' words with spaces ', TRUE],
-      ['words with spaces ', TRUE],
-      [' words with spaces', TRUE],
-      [' 123456 ', FALSE],
-      [' 123456.03 ', TRUE],
-      ['123456.03 ', TRUE],
-      [' 123456.03', TRUE],
-      [' some words & stuff', TRUE],
-    ];
-
-    // Improve test output by using the input value as a label for the dataset.
-    foreach ($data as $index => $item) {
-      // If the index looks like an in eg "1000" "2" etc, php still detects
-      // it as a numeric index. Adding text to all the labels to force them
-      // to render.
-      $data["Sample input: '{$item[0]}'"] = $item;
-      unset($data[$index]);
-    }
-
-    return $data;
-  }
-
-}