Fix bug with for-free passthrough copy: 11ty/eleventy#2456

server.js

@@ -70,7 +70,14 @@ class EleventyDevServer {
 
       let file = this.passthroughAliases[targetUrl];
       if(url.startsWith(targetUrl)) {
-        return file + url.slice(targetUrl.length);
+        let inputDirectoryPath = file + url.slice(targetUrl.length);
+
+        // e.g. addPassthroughCopy("img/") but <img src="/img/built/IdthKOzqFA-350.png">
+        // generated by the image plugin (written to the output folder)
+        // If they do not exist in the input directory, this will fallback to the output directory.
+        if(fs.existsSync(inputDirectoryPath)) {
+          return inputDirectoryPath;
+        }
       }
     }
     return false;