Display allowed extensions and file size limit for attachments on contact forms

Add information about allowed file extensions and file size limit to file fields on the contact forms. If more than one file field is present, wrap them in a fieldset and add the description to it instead of adding the description to each file field.
This change will necessitate figuring out allowed extensions and file size limit in the form builder rather than in the form validation handler, but I don't think this will be significantly bad for performance.