diff --git a/tests/phpunit/includes/functions.php b/tests/phpunit/includes/functions.php
index d27af5c172a7a..473ed341e9571 100644
--- a/tests/phpunit/includes/functions.php
+++ b/tests/phpunit/includes/functions.php
@@ -105,11 +105,19 @@ function _test_filter_build_unique_id( $hook_name, $callback, $priority ) {
 }
 
 /**
- * Deletes all data from the database.
+ * Deletes all data from the database, except:
+ * - The default category.
+ * - The default user.
  */
 function _delete_all_data() {
 	global $wpdb;
 
+	// Retrieve all attachment posts, and delete them along with the attached media.
+	$attachments = $wpdb->get_col( "SELECT ID FROM {$wpdb->posts} WHERE post_type = 'attachment'" );
+	foreach ( $attachments as $attachment ) {
+		wp_delete_attachment( $attachment, true );
+	}
+
 	foreach ( array(
 		$wpdb->posts,
 		$wpdb->postmeta,
diff --git a/tests/phpunit/tests/includes/functions.php b/tests/phpunit/tests/includes/functions.php
new file mode 100644
index 0000000000000..df5fc31a6156e
--- /dev/null
+++ b/tests/phpunit/tests/includes/functions.php
@@ -0,0 +1,33 @@
+<?php
+
+/**
+ * Tests for the `includes/functions.php` file of the Unit Testing Framework.
+ *
+ * @group testsuite
+ * @group phpunit
+ *
+ * @covers ::_delete_all_data
+ */
+class Tests_Includes_Functions extends WP_UnitTestCase {
+	/**
+	 * Verify that attachment files are deleted along with attachment posts.
+	 *
+	 * @ticket 41978
+	 */
+	public function test_delete_all_data_deletes_attachments() {
+		// Create an attachment with an image.
+		$attachment_id = self::factory()->attachment->create_upload_object(
+			DIR_TESTDATA . '/images/waffles.jpg'
+		);
+
+		// Retrieve the path to the image.
+		$attachment_file = get_attached_file( $attachment_id );
+		$this->assertFileExists( $attachment_file );
+
+		_delete_all_data();
+
+		// Verify that the image has been deleted along with the attachment.
+		$this->assertFileDoesNotExist( $attachment_file );
+		$this->assertNull( get_post( $attachment_id ) );
+	}
+}