C#
Surely the likes of Xamarin, Mono, etc make porting C# pretty simple between platforms? Okay, the UI handling might need a bit of tweaking here and there but the backend code should be portable. Plus no Corporate Use license worries as with Java, and with a bit of messing with the interop features you can use existing C/C++ code.
It's a file transfer system, not a game engine or huge distributed AI cluster. Surely there are a multitude of languages and libraries out there for them to use?!
Between this and the other suggestions, it sounds a lot like someone's come in and wants to show off their Nerd credentials rather than looking at the right tool for the job...
Biting the hand that feeds IT
