Tangential to this thread, but your script should work if (changing “/FigTest” to the actual path of your note) you do something like this:
tell application "Tinderbox 10" to set value of attribute "MediaFileName" of (find note in front document with path "/FigTest") to "tinderboxplot.png"
See forum discussion here and explanation of find note in [specifier] with path [text]
in aTbRef here or the AppleScript Dictionary for Tinderbox.