I could be wrong, but I don't think you can assign a custom value to that. It's just the system informing you of the kind of file it is (or thinks it is). I believe the only way to change that would be to change the extension to something other than HTM or HTML, but then, it may not run or open without associating it with a program.
You could associate it with another program capable of reading HTML, such as Firefox, etc.