修复当作业打包下载时，某一文件不存在时，跳过该文件继续下载其他的文件

app/controllers/zipdown_controller.rb

@@ -78,7 +78,7 @@ class ZipdownController < ApplicationController
     # 需要将所有homework.attachments遍历加入zip
     # 并且返回zip路径
     homeattach.attachments.each do |attach|
-      homeworks_attach_path << attach.diskfile#.to_s.slice((length+1)..-1)
+      homeworks_attach_path << attach.diskfile if File.exist?(attach.diskfile)
     end
     zipping("#{homeattach.user.lastname}#{homeattach.user.firstname}_#{((homeattach.user.user_extensions.nil? || homeattach.user.user_extensions.student_id.nil?) ? "" : homeattach.user.user_extensions.student_id)}_#{Time.now.to_i.to_s}.zip", homeworks_attach_path, OUTPUT_FOLDER, true)
   end