Steps to reproduce

# Follow steps in below link to create the image operation: https://docs.magnolia-cms.com/product-docs/6.2/Developing/Templating/Theme.html#_image_variations

1. Upload original JPG image
2. Set the output format to webp

.. Logs, screenshots, gifs...

Expected results

• Output image should be a webp image

.. Justify non-trivial expectations with a link to a doc or a relevant discussion.

Actual results

• A JPG image generated instead

Workaround

• No

  Development notes

• Output format was not correctly resolved in ThemeAwareImageGenerator#90,91. Fix this to have the function work.