Depending on the settings in your email client you may have some difficulty sending HTML files. By default most email clients want to show any attachments that they can successfully convert (text files, images, HTML, etc.)  inline with the rest of your email.  In most email clients there are options to turn off this feature.

In Mozilla Thunderbird the setting can be found under the 'View > Display Attachments Inline' menu item.

Note: This will only change how the messages display for you. To prevent similar behavior in the message the recipient receives you will want to send the HTML file in a compressed folder. This will prevent the recipient's email client from displaying the file inline.

It depends on your goals. If you are trying to send something that is functional (can be used as a form) in some way, you can send zipped HTML. If they have Sesame, you can send XML or a DSR file. If you only want to send an image of the form, you can send a screenshot as an image file.