Page viewer web part problem in Sharepoint 2010

After upgrading SharePoint 2010, the page viewer web parts does not show up. Instead a dialog appears and it tries to download the html file.

This is because of the security settings in Sharepoint 2010
To solve this; you have to navigate to Central administration:

Application Management> Manage Web Applications> select your web aplication & open General Settings

Change "Browser File Handling" to "Permissive.