fix: should only check valid fragments in markdown (.md) files

test/fixtures/valid/only-parse-markdown-files-for-fragments/awesome.html

@@ -0,0 +1,9 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>Awesome</title>
+  </head>
+  <body></body>
+</html>

test/fixtures/valid/only-parse-markdown-files-for-fragments/only-parse-markdown-files-for-fragments.md

@@ -0,0 +1,7 @@
+# Valid
+
+[Link fragment HTML](./awesome.html#existing-heading)
+
+[Link fragment TXT](./abc.txt#existing-heading)
+
+[Link fragment Image](../../image.png#existing-heading)