Customizing logo and layout of the filesharing email templates
1. To use custom email template it should be placed in the whitelabel folder with sub-folder "emailtemplates" and name emailtemplate_BASE_TEMPLATE.html
2. If you want to attach logo as inline image then REPLACE_LOGO_LINK parameter should be used in template and logo should be placed into whitelabel folder with hardcoded name "images/email-logo.gif".
(Inline images is a feature that displays images automatically in email client without the necessity of unblocking them.)
You can find an email template here.