验证是否在codeigniter中选择了要上载的文件

Hi I'm having some trouble about the upload file part. Well basically I have a form with upload logo part wherein if a user doesn't select a logo he/she can still update his/her profile at the same time the user can also choose to upload a logo but it has some limitations. Now I don't know how I can validate that part to know if he/she chooses an image or not. Here's my code so far in CI controller:Hi I'm having some trouble about the upload fil