Thanks for the input FlamingRain. Actually, it was pretty easy to do, I just wanted to make sure I wasn't missing any conditions (as Tiger pointed out, html has 4 letters in the extension).. actually checking for more than one extension only took a few lines of code.

public static void checkDouble()
for (int h=0; h<filePathString.length(); h++)

if ( filePathString.charAt (h) == dot )

count = count + 1;
} // end of if

} //end of for

if (count>=2)
System.out.println("There is a double extension"); // Error marker for later development

} // end of checkDouble

I don't mind creating new classes and methods, I'm sure I'll be able to use them later.