Can I change a file type using Property Calculator?

This might be possible with Property Calculator although I've never tried it, check section 5.8 File Operations in Installing, Configuring, and Using M-Files Property Calculator. You would have to test if the postfix can add the file extension too or if this only applies to the file name before the extension.

hi Joonas, tested and no good result.

the name of a real PDF when dragged to the desktop:
3b354f24 (ID 457).pdf

and the name using postfix:
3b354f24.PDF (ID 206)

Trying to find a way without using vbscript, do you have any other suggestions?

    Dim objFiles:         Set objFiles = Vault.ObjectFileOperations.GetFiles(objVer)
    Dim sFilePath:         sFilePath = "C:\tmp\" & objFiles(1).GetNameForFileSystem() & ".PDF"
    Call Vault.ObjectFileOperations.DownloadFile(objFiles(1).ID, objFiles(1).Version, sFilePath)

hi Joonas, tested and no good result.

the name of a real PDF when dragged to the desktop:
3b354f24 (ID 457).pdf

and the name using postfix:
3b354f24.PDF (ID 206)

Trying to find a way without using vbscript, do you have any other suggestions?

    Dim objFiles:         Set objFiles = Vault.ObjectFileOperations.GetFiles(objVer)
    Dim sFilePath:         sFilePath = "C:\tmp\" & objFiles(1).GetNameForFileSystem() & ".PDF"
    Call Vault.ObjectFileOperations.DownloadFile(objFiles(1).ID, objFiles(1).Version, sFilePath)

I can't think of any other way than custom development if you need to do the renaming after importing. Have you already checked that the renaming cannot be done before importing, either in the source system that is generating these files or with some Windows script in the file system before M-Files reads them in?