diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
index 01e75b9..21b1d92 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapper.php
@@ -891,4 +891,17 @@ public function getMimeTypeForExtension($extension) {
     return isset($extensions[$extension]) ? $this->mapping['mimetypes'][$extensions[$extension]] : NULL;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getExtensionsForMimeType($mimetype) {
+    if (!in_array($mimetype, $this->mapping['mimetypes'])) {
+      return [];
+    }
+    $key = array_search($mimetype, $this->mapping['mimetypes']);
+    $extensions = array_keys($this->mapping['extensions'], $key);
+    sort($extensions);
+    return $extensions;
+  }
+
 }
diff --git a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
index 8618a86..1143aa5 100644
--- a/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
+++ b/core/lib/Drupal/Core/File/MimeType/MimeTypeMapperInterface.php
@@ -43,4 +43,15 @@ public function getMimeTypes();
    */
   public function getMimeTypeForExtension($extension);
 
+  /**
+   * Returns the appropriate extensions for a given MIME type.
+   *
+   * @param string $mimetype
+   *   A MIME type.
+   *
+   * @return string[]
+   *   An array of file extensions matching the MIME type, without leading dot.
+   */
+  public function getExtensionsForMimeType($mimetype);
+
 }