file extensions

[sims]

I am building an import-export feature into an app.

It will export a custom prop or import a plain text
file and make it into a custom prop.

I only want to allow previously exported files to be imported,
so I am thinking of adding a file extension to the file name
as a 'test' to help (ex: only files that end with ".DOG" can be
imported) ensure that only exported files get imported.

I am wondering if this is a reasonable way to proceed...especially
with Windows machines. Type/creator codes for non-osx macs
can be registered, is such a procedure needed for file extensions
on a Windows system?

Thanks in Advance!

sims