Probably the simplest method is to just Import the file(s) into their own table (which you'd like want to do anyway, so you could, if needed, have more than 1 file per order). Then just use a calculation field to extract only the Order Number (which must be unique in the parent Order table). That's really all you need to do to establish a positive relational match to the Order. You'd have a relationship based on this.
If you wanted to further process it, like move it to the Order, you could. But normally I'd say: 1. Not necessary, and, 2. Would block the possibility of > 1 document per order. Unless there can be only 1 per Order.
Alternatively, you could run a FileMaker Find or Find script from the AppleScript. But, since you have a way to process it using the file name, it seems you'd just be making it more complicated (hence breakable; change a field name, Oops! Broke the AppleScript's Find.)