I need to find an archive file that has no more an archive extension

There isn't an built-in way to do that with Opus. Scripting could do it by inspecting the binary data at the start of each file to see if it has the different archive type signatures, at least for archive formats that have them.

You could also use this button and the TrID tool to give any selected files their correct extensions, based on their contents, but you may end up renaming other files as well: