You can do both of these things with CSS Form Builder installed. Does this page submit to itself or another page? You will need to open the page that this form submits to in order to alter the server validations.
When you open the page go to the server behaviors and look for the server validations. Edit this entry and you can identify the validations that you do not need and remove them. You can then add in a new validation for the file field, for the type of validation you should choose 'file extension' and list the allowed file extensions.
This will result in removing the unneeded validations and applying the file extension type validation to your file field. Make sure that you make a backup of the page before you edit it incase you have any troubles.
