The extension .htm will automatically be assigned to a document created in FrontPage.  If your computer has NOT been told to display file extensions, you will want to change the Windows option to show full file names.  You will need to see the extensions for image files, as well as for non-html files you will post.

NOTE:  The options illustrated in this screenshot will vary somewhat with the version of Windows.

On your desktop, click MY COMPUTER | TOOLS| FOLDER OPTIONS.