use the URL being previewed in the Preview link

index.html

@@ -13,7 +13,7 @@
 
         <script id="think-editor-data" type="application/json">
 {
-      "preview_url": "https://somethingorotherwhatever.com/tiny-elvis/",
+      "preview_url": "https://somethingorotherwhatever.com/tiny-elvis",
       "slug": "loaded-thing",
       "files": [
         {"path": ".", "is_dir": true, "name": ".."},

src/App.elm

@@ -339,6 +339,16 @@ link url text =
     [ HA.href url]
     [ H.text text ]
 
+preview_url model = 
+  let
+    file_suffix = suffix model.file_path
+
+    page_suffixes = ["html", "php"]
+  in
+    if List.member file_suffix page_suffixes then
+      model.preview_url ++ "/" ++ model.file_path
+    else
+      model.preview_url
 
 form : Model -> List (H.Attribute Msg) -> List (Html Msg) -> Html Msg
 form model attrs children =
@@ -391,7 +401,7 @@ header model =
       [ HA.id "think-controls" ]
       [ H.a 
           [ HA.target "preview"
-          , HA.href model.preview_url
+          , HA.href <| preview_url model
           ]
           [ H.text "Preview" ]
       , link "rename" "Rename"
@@ -692,17 +702,6 @@ log_pane model =
     ]
 
 preview_pane model =
-  let
-    file_suffix = suffix model.file_path
-
-    page_suffixes = ["html", "php"]
-    
-    preview_url = 
-      if List.member file_suffix page_suffixes then
-        model.preview_url ++ "/" ++ model.file_path
-      else
-        model.preview_url
-  in
   H.details
     [ HA.attribute "open" ""
     , HA.id "preview"
@@ -716,7 +715,7 @@ preview_pane model =
     ++ if model.show_preview then
         [ H.iframe
           [ HA.id "preview-frame" 
-            , HA.src preview_url
+          , HA.src <| preview_url model
           ]
           []
         ]