How to filter multiple extensions in OPendialog

[Himansh]

HI Everybody
I am creating a Software called HTML-Editor inspired from
HTML-IDEx created by brandon

I have also added open button in Menustrips which opens the opendialog1

I have added three extension to it
Webpage(*.html)
Text Files(*.txt)
AllFiles(*.*)

Now you all know that there can be many extension for WEbpage like .html or .htm

Now i want that when user click Webpage extension,
openfiledialog display no only files of extension (.html) but also of extension (.htm)
And i don't want to create a seperate extension for (.htm)

So can anybody tell me add multiple extension to Webpage like
Webpage(*.html,*.htm)
Like you can see in paint
JPED(*.jpeg,*.jpg,*.jpe,*.jpif)

Thanks in Advance !!!!!!!!!