What is an Air-Filed Flight Plan? Are aircraft capable of filing flight plan when in flight?
It is quite easy in the US but it depends on controller workload.  You need to be in a plane equipped for IFR flight and you need to be IFR current....[edit] to legally use a filed IFR flight plan.
If the controller is too busy they will make you file through a flight service station.  This can also be done in flight but requires you to change frequencies, reach the FSS, wait your turn, and then file.  The FSS puts you in the system.  Then you call ATC back and open your plan [edit] -or- open with the FSS.
If the controller has time, they will file you on frequency.