diff --git a/src/wp-includes/customize/class-wp-customize-media-control.php b/src/wp-includes/customize/class-wp-customize-media-control.php
index af137512c41f3..40d644ca84e5e 100644
--- a/src/wp-includes/customize/class-wp-customize-media-control.php
+++ b/src/wp-includes/customize/class-wp-customize-media-control.php
@@ -92,8 +92,9 @@ public function to_json() {
 				 * Fake an attachment model - needs all fields used by template.
 				 * Note that the default value must be a URL, NOT an attachment ID.
 				 */
-				$ext  = substr( $this->setting->default, -3 );
-				$type = in_array( $ext, array( 'jpg', 'png', 'gif', 'bmp', 'webp', 'avif' ), true ) ? 'image' : 'document';
+				$ext       = wp_check_filetype( $this->setting->default )['ext'];
+				$ext_types = wp_get_ext_types();
+				$type      = isset( $ext_types['image'] ) && in_array( $ext, $ext_types['image'], true ) ? 'image' : 'document';
 
 				$default_attachment = array(
 					'id'    => 1,
diff --git a/tests/phpunit/tests/customize/media-control.php b/tests/phpunit/tests/customize/media-control.php
new file mode 100644
index 0000000000000..3a2056f96baa6
--- /dev/null
+++ b/tests/phpunit/tests/customize/media-control.php
@@ -0,0 +1,63 @@
+<?php
+/**
+ * Tests for the WP_Customize_Media_Control class.
+ *
+ * @package WordPress
+ *
+ * @coversDefaultClass WP_Customize_Media_Control
+ *
+ * @group customize
+ */
+class Test_WP_Customize_Media_Control extends WP_UnitTestCase {
+
+	/**
+	 * Set up.
+	 */
+	public function set_up() {
+		parent::set_up();
+		require_once ABSPATH . WPINC . '/class-wp-customize-manager.php';
+	}
+
+	/**
+	 * @ticket 64557
+	 *
+	 * @covers ::to_json
+	 */
+	public function test_to_json() {
+		$manager = new WP_Customize_Manager();
+
+		$manager->add_setting(
+			'some_jpg',
+			array(
+				'default' => 'https://example.com/image.jpg',
+			)
+		);
+		$manager->add_setting(
+			'some_avif',
+			array(
+				'default' => 'https://example.com/image.avif',
+			)
+		);
+		$manager->add_setting(
+			'some_pdf',
+			array(
+				'default' => 'https://example.com/image.pdf',
+			)
+		);
+		$manager->add_setting( 'no_default' );
+
+		$some_jpg_control   = $manager->add_control( new WP_Customize_Media_Control( $manager, 'some_jpg' ) );
+		$some_avif_control  = $manager->add_control( new WP_Customize_Media_Control( $manager, 'some_avif' ) );
+		$some_pdf_control   = $manager->add_control( new WP_Customize_Media_Control( $manager, 'some_pdf' ) );
+		$no_default_control = $manager->add_control( new WP_Customize_Media_Control( $manager, 'no_default' ) );
+
+		$some_jpg_control_json  = $some_jpg_control->json();
+		$some_avif_control_json = $some_avif_control->json();
+		$some_pdf_control_json  = $some_pdf_control->json();
+
+		$this->assertSame( 'image', $some_jpg_control_json['defaultAttachment']['type'] );
+		$this->assertSame( 'image', $some_avif_control_json['defaultAttachment']['type'] );
+		$this->assertSame( 'document', $some_pdf_control_json['defaultAttachment']['type'] );
+		$this->assertArrayNotHasKey( 'defaultAttachment', $no_default_control->json() );
+	}
+}