Please add $pagefile->isImage property or method

[BernhardBaumrock]

Short description of the enhancement

I'm listing files uploaded by the client to a files field. For images I want to schow a preview image. Now I need to check for all possible file extensions, which should better be done once in the core instead of in every project of me and everybody else :)

If that's already possible I'm sorry :)

[BernhardBaumrock]

I've added it to RockMigrations via hook in the meantime: https://github.com/baumrock/RockMigrations/commit/43a06de5863cb49a6ed4429dd174fa1d6ae16792