Assuming you are running on Linux you can use executeCommandLine:
val fileExists = executeCommandLine("ls /etc/openhab2/transform/"+roomName+".map", 1000)
val mapFile = if(fileExists.endsWith("No such file or directory")) roomName.map else "Default.map"
In JSR223 Python you can use:
from os import path
...
mapFile = ("{}.map".format(roomName)
if path.exists("/etc/openhab2/transform/"+roomName+".map")
else "Default.map")
You can use the native Java classes:
import java.io.File
...
val f = File("/etc/openab2/transform/"+roomName+".map")
val mapFile = roomName+".map" if(f.exists()) else "Default.map"
If using JSR223 Rules, you can instead encode the map files on the Items themselves as metadata and potentially sidestep this issue entirely.
I will incoroperate the java version, because that is then platform independent in my light tutorial. I will update the tutorial with this new feature and let you know about it.