Importer warning is not shown right after invalid file drag and drop

Comes from the [comment](https://github.com/Automattic/simplenote-electron/issues/2777#issuecomment-872175459) to #2777

### Expected
During the import of an unsupported file type via drag and drop, there should be an immediate warning.

### Observed
The warning is shown only after the valid file import, and the size of the warning also breaks the dialog height:

https://user-images.githubusercontent.com/73365754/124119146-d1767a00-da7a-11eb-86c7-db65e0128300.mov

### Reproduced
1. Open import dialog
2. Drag and drop an unsupported file type (e.g. an image) ~> visually nothing will happen
3. Now drag and drop a valid file (e.g. a text file) ~> you will the valid file being accepted by importer, but also the warning for the image file, as well as the unnecessary scrollbar.

### Where did you see the bug
- OS: Windows 10 Home | Parallels Desktop
- Simplenote app version: Desktop 2.14.0-beta1

Reproduce the issue in the import dialog by dropping an unsupported image and then a valid text file, as described. Trace the drag-and-drop importer handling; done means the unsupported-file warning appears immediately and the dialog does not gain an unnecessary scrollbar.

Written by the indexing model from the issue text.