Yes, I have customised the Media Property Box, but NOT the (Linked) File item.
That would explain why its label has changed from Linked File to File in my installation.
It is all very strange.
I have inspected the C:\Program Files (x86)\Family Historian\Property Box\PB-OBJE.fhdata file dated 24/06/2013 and it defines the File label.
The same goes for C:\ProgramData\Calico Pie\Family Historian\Property Box\PB-OBJE.fhdata file that is identical.
But there is also an old FH Property Box - Main tab for Multimedia.fhpbt file dated 10/04/2012 that defines the Linked File label.
So it looks like the definition files changed format and labels during FH V5 but the new form only takes effect after a Property Box customisation!