i cant say why it is trying to save as an html file, if you go to file save, Dreamweaver should not try to change the files extension.
there is one basic form principle we keep coming back to:
All Form Processing MUST be performed on the forms action page.
the way you have it set up right now will not work.
Right now, the server validation is on the form page, and the form is set to post to the submit page.
This means the validation will be skipped and the email will be sent no matter what.
if the validation behavior and the email behavior are not on the same page it will not work
here is how i would go about doing this:
1) create a new file, add your contact form. make sure the method is set to POST and leave the action blank.
2) Apply the captcha element, then add the server validation
3) Add the Universal Email behavior. Select the option to Create a file for the email message, and select one of the templates (This is why your email is blank, you didn't do this before), you can set universal email to redirect to another page.
this way the form, captcha, validation and UE are all on the same page.
the behaviors should apply them selves in the correct order