// ...
@Component({ ... })
export class MyComponent {

  file: any;

  constructor() {}

  getFile(event: any) {
    this.file = event.target.files[0];

    const classe = {
      libelle: null,
      ecole: null,
      date_creation: null,
      nbre_etudiant: null
    };

    this.fileReader(this.file, classe);
  }
  // ...
}