Create a new file without the prefixed unique identifier?

Can I create and save a new file without it automatically prefixing a unique identifier?

I create a thumbnail from a file and it would be nice if I can just name the thumbnail “thumb_xxxx”, where xxxx is the original file’s name.
By having the download URL of the original file it is simply a matter of appending the “thumb_” part to get the download URL of the thumbnail.

I found the preserveFileName server option but I would like it to only preserve it when saving a file in the cloud code.

Could you add a beforeSaveFile trigger in cloud code and conditionally add the prefix?

But the problem is that even if I set the filename in beforeSaveFile the filename is correct, e.g. “thumb_xxx”, but once in afterSaveFile the filename becomes randomstring_thumb_xxx.