The new files preview presents unsupported file types in an unfriendly fashion.

When an unsupported file type is attached to a page, clicking on a link attempts to preview the file, with an error message "This file cannot be previewed". Prior to 5.7, these files would simply be downloaded. Now, users get an extra pop up which may be confusing. Users can still download the file from the top right corner overlay, but may not remember to do so.

There are a few points concerning this file preview:

1. If the file can't be previewed (and we should have a good idea based on it's mime type and extension), why do we not just allow the file to download?
2. If we can't preview the file, the message that states the file cannot be previewed should help the user to an alternate action - in this case, to download the file. Either a direct link or a guide to the download icon in the top right corner to foster the correct behavior. The current preview window relies on the user knowing the behavior of this window, which may not be 100% concrete so soon after an upgrade

Alternatively, it would be awesome if an administrator could control what file types are attempted to be previewed or not. If for example, we add the ability to preview visio files, and a company with large visio files may not want them previewed in their instance, they should have the ability to disable previews for a certain file type - and just allow the file to be downloaded.