How to access a value for file metadata before upload in keystone schema?

Question

I am currently using keystone js to build a small admin panel for my website i am trying to check the type of the file before adding it to the database so i have to access the mimetype to validate the type and validate the input in keystone's Schema.ts

{
  upload: Promise {
    {
      filename: 'Ahmed_M Salah_Resume_11-02-2023-14-46-38.pdf',
      mimetype: 'application/pdf',
      encoding: '7bit',
      createReadStream: [Function: createReadStream]
    }
  }
}