When path to export is in the form if "/some/folder/.stl", any
regular path functions (BLI_path_suffix, BLI_path_extension_replace
etc.) would not treat it as having ".stl" extension, but rather
as a hidden file called ".stl".
And so BLI_path_suffix would turn it into "/some/folder/.stlCube".
To prevent this, detect this exact case when the whole basename
is just ".stl", and remove the ".stl" part before using other path
functions.
Pull Request: https://projects.blender.org/blender/blender/pulls/125179
fe1bf4897d