Final Draft 7 is not supported on OS X 10.12 (Sierra), 10.11 (El Capitan), 10.10 (Yosemite), 10.9 (Mavericks), 10.8 (Mountain Lion) or 10.7 (Lion).
While OS X allows for longer file names, earlier Mac operating systems don't. Because Final Draft 7 and 6 run in the earlier operating systems as well as OS X, the program conforms to those systems for the length of the file name, which is why it won't allow you to extend the file name. If you change the file name outside the program, it won't recognize the long string of characters and will shorten it this way.
Note: Final Draft 10 / 9 / 8 support long filenames. Information on the upgrade to Final Draft 10 is HERE.
NOTE: All support for Final Draft 7 and older, including software activation, has been discontinued.