You could try this script, I think it will do the trick:
Set Error Capture [on]
Insert File[//specify a container field, by reference, and the filename and path to the file--using a variable if you need to]
If [ Get ( LastError ) = 0 /* file exists */]
Else // it does not
Hmm...seems like it should work, but in the compiled program (it's commercial software, so it has to work in the compiled version), despite the specificity of the file name in the Insert File command, what winds up getting stuffed into the Container field is not the file I specified in the script step, but the current USR file, so it always passes the test. I haven't as yet tried to debug it running from the FMP environment...
Not sure why you are getting that result in the Runtime, but it doesn't work smoothly in my test with regular FileMaker anyway. If the file doesn't exist, I get an open file dialog looking for the file and that won't work for what you want here.