Modifying a .htm through word makes it a word file???

I've got a project that we are completely revamping. We were hoping to be able to send the individual .htm files to several subject matter experts to have them edit the content...then just copy/paste the .htm files back into the project. When we open the project the files still have a .htm extension but the application tied to them is Word. When you access a newly copied file it opens word and makes you edit it there. Anyone have a suggestion on a better way to approach this...we want to work smart not hard.

Thanks!