fix library form cover

app/views/libraries/_form.html.erb

@@ -73,7 +73,7 @@
             <div class="surfacePlot ml25 library-cover-select" data-tip-down="上传图片" onclick="$('#library-cover-file').click();">
               <img src="/images/educoder/unite.png" class="marginuploading <%= cover_exists ? 'hidden' : '' %>" style="<%= cover_exists ? 'display:none' : '' %>"/>
               <div class="library-cover-perview" style="<%= cover_exists ? '' : 'display:none' %>">
-                <% cover_url = cover_exists ? named_attachment_path(@library.cover.id, @library.cover.filename) : '' %>
+                <% cover_url = cover_exists ? download_attachment_path(@library.cover_id) : '' %>
                 <img src="<%= cover_url %>" width="120px" height="90px"/>
               </div>
             </div>