diff --git a/src/manifest.json b/src/manifest.json
index b4b4401..a8aefd8 100644
--- a/src/manifest.json
+++ b/src/manifest.json
@@ -1,10 +1,10 @@
 {
   "name": "Edit in Prose",
   "description": "Provides an \"Edit in Prose\" link from a hosted file on GitHub.",
-  "version": "1.3.0",
+  "version": "1.4.0",
   "manifest_version": 2,
   "author": "Tristen Brown",
-  "homepage_url": "http://prose.io",
+  "homepage_url": "https://prose.io",
   "icons": {
     "16": "icon16.png",
     "48": "icon48.png",
diff --git a/src/prose.js b/src/prose.js
index b6a5c54..1ca61a3 100644
--- a/src/prose.js
+++ b/src/prose.js
@@ -9,32 +9,44 @@
     const repo = parts[1];
     const branch = parts[3];
     const path = parts.slice(4).join('/');
+    const configFilePath = 'https://github.com/' + user + '/' + repo + '/blob/master/_prose.yml';
 
+    // User test
     if (!user || !/^\w[\w-]+$/.test(user)) {
       return;
     }
 
-    // Nav button
-    if (repo) {
-      const nav = document.querySelector('.hx_reponav');
-      const button = document.querySelector('#prose');
-      if (!button) {
-        const link = document.createElement('a');
-        link.id = 'prose';
-        link.innerHTML = '<span class="prose-icon"></span>Prose';
-        link.title = 'Open in Prose';
-        link.className = 'js-selected-navigation-item reponav-item';
-        link.rel = 'nofollow';
-        nav.appendChild(link);
-        link.href = 'http://prose.io/#' + user + '/' + repo + '/';
+    // Prose config file test
+    fetch(configFilePath).then(response => {
+      if (response.ok) {
+        // Display nav button
+        const nav = document.querySelector('.hx_reponav');
+        const button = document.querySelector('#prose');
+        if (!button) {
+          const link = document.createElement('a');
+          link.id = 'prose';
+          link.innerHTML = '<span class="prose-icon"></span>Prose';
+          link.title = 'Open in Prose';
+          link.className = 'js-selected-navigation-item reponav-item';
+          link.rel = 'nofollow';
+          nav.appendChild(link);
+          link.href = 'https://prose.io/#' + user + '/' + repo + '/';
+        }
+      } else {
+        throw new Error('Can\'t find "_prose.yml" file');
       }
-    }
+    }).catch(err => console.log(err.message));
 
-    // Sha Test
+    // Sha test
     if (/^[0-9a-f]{40}$/.test(branch)) {
       return;
     }
 
+    // Markdown file type test
+    if (!/\.markdown|mdown|mkdn|mkd|md$/i.test(path)) {
+      return;
+    }
+
     // Edit button
     const root = document.querySelector(rootClass);
     if (!root) {
@@ -57,6 +69,6 @@
       edit = link.firstChild;
       group.insertBefore(link, group.firstChild);
     }
-    edit.href = 'http://prose.io/#' + user + '/' + repo + '/edit/' + branch + '/' + path;
+    edit.href = 'https://prose.io/#' + user + '/' + repo + '/edit/' + branch + '/' + path;
   }
 })();