Both "html" and "htm" are standard file extendsions for HTML files, but only "html"/"HTML" is recognized.

test cases:
http://dstress.kuehne.cn/run/html_htm_01.htm
http://dstress.kuehne.cn/run/html_htm_02.HTM
http://dstress.kuehne.cn/run/html_html_01.html
http://dstress.kuehne.cn/run/html_html_02.HTML

Thomas