You profile doesn't say if you are on Windows or Mac. If the latter you could create a folder action script. This has to be an Applescript. That script could run a Filemaker script to import the file.
How are you connecting between the Arduino and the computer running Filemaker? I assume Wifi. However you connect, overwrite the file in the folder each time a weighing occurred to trigger the folder action.
Just checked, Windows does have folder actions.