PatchValue for Upload file in angular

So i have edit form page like this

and form a group like this

this.editMateri = this.fb.group({
  name: [''],
  description: [''],
  thumbnail: [null],
  isPublish: this.status,
  materialCreatorCode: [''],
  tags: this.selectedTags
});

and i fetch the data from the server and set the value to the form using patchValue like this

this.editMateri.patchValue(this.currentMateri);

everything works fine, but the problem is with the thumbnail File Upload input, by default it's null so if i update anything except the thumbnail, my thumbnail will goes blank, the thumbnail is set to null. I've been looking for stackblitz code for this but i didn't find any, anyone can help me?

EDIT

in my html like this

<div class="form-group">
    <label for="exampleInputEmail1" class="label">Thumbnail</label>
    <input type="file" nbInput fullWidth (change)="uploadFile($event)">
</div>

and for uploadFile function like this

  uploadFile(event) {
    const file = (event.target as HTMLInputElement).files[0];
    this.editMateri.patchValue({
      thumbnail: file
    });
    this.editMateri.get('thumbnail').updateValueAndValidity()
  }