问如何验证文件内容类型为pdf，word，excel和纯文本的回形针？

EN

 has_attached_file :uploaded_file,  
                      :url => "/policy_documents/get/:id",  
                      :path => "/public/policy_documents/:id/:basename.:extension" 

    validates_attachment_size :uploaded_file, :less_than => 10.megabytes    
    validates_attachment_presence :uploaded_file 
     validates_attachment_content_type :uploaded_file, :content_type =>['application/pdf', 'application/xlsx'],
                                                       :message => ', Only PDF, EXCEL, WORD or TEXT files are allowed. '