“Can't open the file because the file format/extension is not valid.” error on website/desktop app.

Thanks for posting on our Community, Mitz1​. 

First off, I’d like to clarify that we’re not the support team (your support options may differ based on your plan) and we can’t see account-specific info here on the forum. There are still a few things to check together though, in order to determine what may have happened.

Now, can you clarify if you can see the Dropbox icon in your system tray/menu bar at the moment? If the Dropbox app seems to be syncing now, I’d like to know its syncing status.

Other than that, I’d like two separate screenshots of the files you can’t open; one screenshot of what you see on your Dropbox account online and one more of what you see on your desktop app (including any sync icons you may be seeing next to those files).

In general, Dropbox allows you to recover previous versions of files for a specific period of time, which depends on your current Dropbox plan. However, I’d like us to gather a few more details first, to isolate the issue.

Let me know once you have more info.