IDM Automatically Changing File Extension From Default

IDM (version 6.25) is changing certain file types, eg m4b to mp4, tgz to gz. Downloading through a multi filehoster works fine, it’s only when I use IDM to download off the internet/direct http access. Downloading through the web browser or JDownloader works fine.
I’ve tried adding those extensions, as well as *.* in the “File Types” (and restarting IDM) but it didn’t seem to do anything.
I’ve tried googling this but I can’t find anything.
Any help would be most appreciated.
Answer #1
I’d say it’s being done by the OS during the save action – do you have .tgz assigned to a program?
If not, create the reg assignment (run winrar and use its settings to assign tgz to it) – after that IDM should save the file as tgz. The same might apply to m4b; an unassigned extension often gets renamed to a nearby, known, “same type” extension.
Answer #2
That makes perfect sense, thanks for that. Unfortunately it didn’t work for m4b, it still changes it to mp4. But I’m not worried since you mentioned it gets renamed to a “same type” extension. The file seems to work as normal anyway.