I might have expected to see this if you used .DisplayName, which I understand respects the preferences of Explorer
(Hide extensions for known file types)
When you say ‘change the name to anything other than’… do you mean pot.tfx works?
If so, does prn.wibble show the same problem?
And if it does, it will be because PRN is being considered like a device name … its an old shorthand for the printer device.
If you try to create a file with that name in the Windows Explorer the system will say “The specified device name is invalid.” When you use the .wobble extension you are not relying on the old 8.3 subsystem of filenames and thus can create the file.
Correction: you cannot create the file.
From the article these are all the names that should be avoided:
CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9. Also avoid these names followed immediately by an extension; for example, NUL.txt is not recommended.
Using a filename of PRN with any three-letter extension should result in an error. The reason being that PRN is a MS-DOS device name for the default system printer.
In the old days we could open “PRN” as a filename and write text data to it, that data would then be printed.
Using more than three letters (like .wobble) for the extension changes the format of the 8.3 filename created on the disk, most likely PRN~1.WOB, which is allowed.