Fix crash when adding file attachment to problem (#258)

Previously, if there was a validation error when adding a file
attachment to a problem (e.g. "Filepath has already been taken",
"Filepath extension doesn't match file"), then instead of displaying
the validation error message, there would be a 500 Internal Server
Error (First argument in form cannot contain nil or be empty). Full
trace below.

The cause was an incomplete renaming in commit 1e93118 (enable file
attachments for problems, with shared code for filelinks of groups and
problems, 2013-12-19). The code path that triggers when there is a
validation error still used some old variable names, causing the
crash.

Fix by updating those variable names.

Full error trace:

```
Completed 500 Internal Server Error in 764ms (ActiveRecord: 64.1ms)
First argument in form cannot contain nil or be empty excluded from capture: DSN not set

ActionView::Template::Error (First argument in form cannot contain nil or be empty):
    1: <%= form_for @new_filelink, :url => index_path do |f| %>
    2:   <% if @new_filelink.errors.any? %>
    3:     <div id="error_explanation">
    4:       <h2><%= pluralize(@new_filelink.errors.count, "error") %> prohibited this post from being saved:</h2>
  app/views/filelinks/roots/index.html.erb:1:in `_app_views_filelinks_roots_index_html_erb___4252713757973532291_70289880'
  app/controllers/filelinks/roots_controller.rb:88:in `create'
```

app/controllers/filelinks/roots_controller.rb

@@ -80,11 +80,11 @@ def create
     self.model = load_model
     authorize model, :update?
     authorize FileAttachment.find(filelink_params[:file_attachment_id]), :use?
-    @new_file = model.filelinks.build(filelink_params)
-    if @new_file.save
+    @new_filelink = model.filelinks.build(filelink_params)
+    if @new_filelink.save
       redirect_to(index_path, :notice => "File attachment added")
     else
-      @files = model.filelinks.order(:filepath)
+      @filelinks = model.filelinks.order(:filepath)
       render :action => :index
     end
   end