Move changeExtension logic into manipulate method

[afbora]

Changelog

Fixes

• File create.format blueprint option also converts mp4 to broken webp #6715

Breaking changes

None

Docs

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add lab and/or sandbox examples (wherever helpful)
• Add changes & docs to release notes draft in Notion

[distantnative]

@afbora Should we maybe also have a unit tests for the regular methods? E.g. testCreateWithManipulateForNonImage?

[distantnative]

@afbora I think we misunderstood each other. My comment was not about the naming of "non image". My suggestion was to not just test the manipulate method in isolation, but actually tests methods like e.g. create with blueprint create option and a non image to ensure that they now really remain untouched.