Inline comments, signature HTML template: Add hints for inline images

[andreashaerter]

It is not obvious how to create usable data-URLs for HTML signatures in Roundcube. An example as addition for inline comments / documentation in config.inc.php.dist for $config['identity_from_directory_signature_template_html'] or the README would be nice.

[andreashaerter]

Easiest way is a base64 data URL. Sadly, Gmail does not support this (all other important clients and Web UIs do): https://www.caniemail.com/features/image-base64

However, most compatible way would be an attachment with CID like described at https://stackoverflow.com/a/9110164. Roundcube supports this and does it automatically when using the web UI by passing a stored image URL, converting it into an CID / attachment when creating an email. Triggering the mechanism via template is not a real problem (a certain blog: URL does so) but how to prepare the image globally from a plugin config value?