Mac and Dropbox icloud synchronization mass file corruption

Hi, I have lost virtually all of the files (over 1,000 files) on my Mac from using both Dropbox and Mac storage. Mac tech support blames Dropbox for this (I am not in contact with engineering directly). Meanwhile the Dropbox engineer says it has no control over the Mac storage.

What is happening is that the files get a .icloud extension added to the original file which becomes, for example, .key.icloud. Dropbox has now been uninstalled but the files still fail to open. Some have 'zero bytes' while some have, for example, 82.4mb showing.

When I ask for an explanation, I get stonewalled. Can anyone explain what is happening and why? I am now too scared to create or use files on the Mac as they corrupt as soon as they are closed.