codePreviews: ignore non-plaintext

src/modules/codePreviews.js

@@ -23,6 +23,7 @@ function unindent(str) {
 async function processFile(link) {
   const res = await fetch(link);
   if (!res.ok) return "";
+  if (!res.headers.get("Content-Type").startsWith("text/plain")) return "";
   const file = await res.text();
   const lines = file.replace(/\r/g, "").split("\n");