[RTE]: Restrict ability to upload not PDF files via iframePlugin

**Description**
On Windows systems even if `accept` attribute is set you are still able to upload a file of any extension you like. You can achieve it by choosing "All Files (*.*)" option in opened File Explorer window. Unfortunately, wrong file extensions may lead to unexpected behavior.

**Describe the solution you'd like**
We have managed to avoid this case by `iframePlugin` customization. If we receive a file of a wrong format, we show a warning modal. But we are convinced that this restriction should be implemented within UUI library itself.