{{ image_data|data_uri }}

{{ source('path_to_image')|data_uri }}

{# force the mime type, disable the guessing of the mime type #}
{{ image_data|data_uri(mime="image/svg") }}

{# also works with plain text #}
{{ 'foobar'|data_uri(mime="text/html") }}

{# add some extra parameters #}
{{ 'foobar'|data_uri(mime="text/html", parameters={charset: "ascii"}) }}