Fix showing notification about unsupported files when `clipboard_handleImages` is OFF

[KarolDawidziuk]

What is the purpose of this pull request?

Bug fix

Does your PR contain necessary tests?

All patches that change the editor code must include tests. You can always read more on PR testing, how to set the testing environment and how to create tests in the official CKEditor documentation.

This PR contains

• [x] Unit tests
• [x] Manual tests

Did you follow the CKEditor 4 code style guide?

Your code should follow the guidelines from the CKEditor 4 code style guide which helps keep the entire codebase consistent.

• [x] PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

* [#5431](https://github.com/ckeditor/ckeditor4/issues/5431): Fix: No notification is shown for unsupported image types.

What changes did you make?

.

Which issues does your PR resolve?

Closes #5431.