"there was a problem previewing this document" in Gmail

I solved it reenabling Flash option in Chrome settings (for mail.google.com site)

The Disconnect Chrome extension causes this. The easiest is to disable it for Gmail (the same goes for Inbox).

Just click og the disconnect plugin next to the address bar and the click on Whitelist site. This reloads Gmail and the attachment should now be available. After clicking Whitelist site the text changes to Blacklist site.

This related to security settings which IT department of companies set in order to make documents handling more secure. Contact your IT department. They should be able to provide a solution to this. It has got nothing to do with Google figuring out what is secure/ not secure documents.

Another option is if you have another email provider, like Yahoo for example, you can send the email to your Yahoo account and open the attachment there.

That means the attachment you are previewing is not a valid attachment for being able to open by Google apps (like document, excel sheet, zip file, image, etc.). If any attachment is not considered as secure for security reasons then in that case also, Google won't make it available for preview. The only remaining option that Google can do for you in this situation is to let you download so that you may find some other tools to open that attachment.