因為一些原因，需要驗證使用者是否有上傳照片，若有上傳照片才需要驗證檔案大小、檔案格式
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| # model.rb
validates_attachment_size :avatar, {
less_than: 300.kilobytes, if: Proc.new{|imports| imports.avatar.uploaded_file.present?}
}
validates_attachment_content_type :avatar, {
content_type: ["image/png", "image/jpeg", "image/jpg"],
if: Proc.new{|imports| imports.avatar.uploaded_file.present?}
}
|