Fix: Change the file extension to .3sh and it should work.
Explanation: The standard 3D shapes in Stunts are in a compressed file format (or "packed", hence the "p" prefix in many extensions). stressed, however, only saves unpacked files. Stunts can load both compressed and uncompressed files, as long as the extensions are correct (the common correspondences are .p3s -> .3sh , .pvs -> .vsh and .pre -> .res ; the Wiki has a complete list
). Incidentally, one of the latest patches to stressed (set for the next release) changes the behaviour of the save dialog to prevent the sort of issue you had.
P.S.: If you are going to use a .3sh or .vsh file, be sure that there is not a .p3s or .pvs file with the same name in the Stunts folder, as given both choices the game will pick the compressed file.