I guess, it may be not Add-On issue.
I think "All Add-Ons" is just open dialog filter name.
So, Tool Add-On extension is "apx".
Lets you type "myaddon.dll" and the file exists.
I think you got this message as extension is not "apx".
And it is not "All Add-On" 🙂
So, check extension of your Add-on, move it into empty folder (for any case, may be some file with same name but other extension is the issue).
And try.