Due to potential security issue SVG theme upload are disable by default.
You need to allow it manually via config.php file

https://github.com/LimeSurvey/LimeSurvey/blob/bbdb4dfa8ec79774660973280dff7abb7dbbae0f/application/config/config-defaults.php#L98
https://github.com/LimeSurvey/LimeSurvey/blob/bbdb4dfa8ec79774660973280dff7abb7dbbae0f/application/config/config-defaults.php#L100

Maybe an issue for manual ?
Ping @tibor.pacalat

Move it to manual/documentation issue ?

https://manual.limesurvey.org/index.php?search=SVG

Or maybe :

Steps :

1. Take the vanilla template and extend it.
2. Upload the SVG file.

Expected result

Show n error or warning

by using {{ image('files/img.svg') }} you get back empty string. I guess is the same issue. Also {{ imageSrc() }} is not working with SVG files.
Why should be svg insecure as logo / template file? (ok for regular upload)

https://github.com/LimeSurvey/LimeSurvey/blob/bbdb4dfa8ec79774660973280dff7abb7dbbae0f/application/core/LSYii_ImageValidator.php#L56-L65

I whitelisted svg in config and {{ imageSrc() }} started working. I lost one day debugging this undocumented issue.

When file is SVG, {{ image() }} should render as

            <svg >
                <use xlink:href="{{ imageSrc() }}"></use>
            </svg>

by using {{ image('files/img.svg') }} you get back empty string. I guess is the same issue. Also {{ imageSrc() }} is not working with SVG files.

Yes still undocumented https://www.limesurvey.org/manual/index.php?search=SVG

Added : https://www.limesurvey.org/manual/Optional_settings#Allowed_file_upload

Added : https://www.limesurvey.org/manual/Optional_settings#Allowed_file_upload

I contributed with a few lines. I also added notes in twig theming documentation https://www.limesurvey.org/manual/Theme_editor_-_Available_Twig_functions#Image_and_file_source

Thank you both! Is there still something to be done for this issue, or I can close it?

Why not move to issue with :

1. show a proper error if file type is invalid
2. AND/OR Restrict input file to https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/file#limiting_accepted_file_types ?

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=41203