Add test for fileTypes option

tests/OptionsTest.php

@@ -0,0 +1,26 @@
+<?php
+declare(strict_types = 1);
+
+use PHPUnit\Framework\TestCase;
+use LanguageServer\Options;
+
+class LanguageServerTest extends TestCase
+{
+    public function testFileTypesOption()
+    {
+        $expected = [
+            '.php',
+            '.valid'
+        ];
+
+        $options = new Options;
+        $options->setFileTypes([
+            '.php',
+            false,
+            12345,
+            '.valid'
+        ]);
+
+        $this->assertSame($expected, $options->fileTypes);
+    }
+}