I haven't found any documentation that specifies the program must have a .txt extension. I've seen .prg mentioned, however. On the 70/700 series you can use a wildcard (*.*) to transfer a file from the FD, but I didn't see anything about changing the default extension.
What extension does Bobcad tack on the the code files? Can't the programmers specify to use a .txt extension when they create the code? At least to get you going?