Image upload for Thumbnail doesn't work properly

Steps to reproduce:

1. Log in as Admin
2. Add a title to create a new blog post
3. Add some text in Content and Summary
4. Click on Choose an image
5. Click on Browse (select an image with format: png, jpg, gif - Open)
6. Click on Upload and select

Expected results: The image selected is uploaded and selected, the Attachments box disappears.

Actual results: On all browsers except Chrome a new message box appears when clicking on the Upload and select button asking you if you want to leave or stay on the current page. You have to click on Stay on this page (the Upload still appears at the bottom of the page) and click again on the Upload and select button for the image to work.