This affects our company as well with Confluence (LTS) Version 7.4.8 which is the most actual on to this date. Our users reported this also with xlsx and pptx files. 

This is a serious issue and makes document management inconvenient as it this is a major feature of Confluence.
As there is no one assigned and there has not been a comment from Atlassian should users assume this is not on the backlog?

I get around this by creating a button that simply does a direct download (no preview). Using the UI Button macro provided for free to Confluence server as part of the Refined Toolkit for Confluence.

1. Attach your document to the page via the attachment window. .i.e. don't include it as a visible part of the page.
2. Right click and copy the link to the attachment
3. Edit page and add a UI Button macro with the URL from the attachment

Clicking the button on the page will direct download whatever file you linked.

Still a workaround to a bug, but it gets the job done. 

My users are still facing this issue in Confluence 7.4.5 (e.g. with JSON Attachments).

The workaround has been deleted form Bitbucket. We are having the probleme only with XLSX files. Any hints?

this has been reported for more than a year ago. How long time does it take to verify this bug?

this is a bug, not an improvement. With Atlasssian significantly increasing the support cost of the product, we should receive better support. It appears the consensus from the comments is in agreement. Please change from improvement to bug and provide a date for resolution. This is holding up our upgrade.

As mentioned in Answers:
It's nice to have new "features" but when @atlassian implements such features a Admin (like me) should be able to at least disable them without tempering the source code :/

Atlassian, I am also facing the same problem after upgrading my instance from 5.6.6 to 5.7.1 where I am unable to preview the PDF files from the space.

Please provide a workaround for the same.

Atlassian, this is a bug, not an improvement. If I provide a fancy feature that only works in certain cases and fails in an unfriendly way in other cases, it is a regression from the previous behavior in which the feature didn't exist at all. Therefore it is a bug. To get back to old status quo for the cases where the feature doesn't work, you need a bug fix, not an improvment.