Visualforce CSV Example. Note the 'contentType' page attribute and the # to specify the file name.

VisualforceCsvExample.html

  <!--
    This page must be accessed with an Account ID in the URL.
    For example: https://<salesforceInstance>/apex/ContactsCsvPage?id=001D000000JRBet
    https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_styling_content_type.htm
-->
<apex:page standardController='Account' contentType='application/vnd.ms-excel#contacts.xls'>
    <apex:pageBlock title='Contacts'>
        <apex:pageBlockTable value='{!account.Contacts}' var='contact'>
            <apex:column value='{!contact.Name}'/>
            <apex:column value='{!contact.MailingCity}'/>
            <apex:column value='{!contact.Phone}'/>
        </apex:pageBlockTable>
    </apex:pageBlock>
</apex:page>

Code snippet for my blog post about the many ways to export Salesforce data to CSV: https://douglascayers.com/2016/03/20/salesforce-easy-ways-to-export-data-as-csv/