Allow unknown file extensions to be selected in file open dialog

[rasteiner]

This PR …

Resumes the work of #4344, but without merge conflicts.

It:

• Implements the acceptAttribute method in the FileBlueprint class.
• deprecates the acceptMime method
• adds a $matchWildcard attribute to the Kirby\Filesystem\Mime::toTemplates() method.
• changes the upload field mixin and the files section config to use acceptAttribute instead of acceptMime.

The new acceptAttribute function returns a list of extensions, instead of the list of mime types.

The primary merit is that this allows files to be uploaded which are unknown to Kirby, but without forwading an all allowing * to the browser file open dialog. Because Kirby and the browser no longer need to know a mime type for those files.

The modified Mime::toTemplates method can now produce an array of extensions also for a mime type like image/*.

Breaking changes

I'm not sure if the deprecation is a breaking change, in which case we could just not deprecate the no longer used toMime() method.

Docs

acceptAttribute also has a clearer docblock which mentions how the list of file extensions is actually built:

• If there is a mime type, the "extension" and "type" options are ignored.
• If there is no mime type, only the intersection between the "extension" and "type" is returned (not their union).
  The reproduces how acceptMime worked.

Ready?

I think so :)

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

For review team

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

[distantnative]

Thanks for adding the changes. I think @lukasbestle should have the final review.

[lukasbestle]

I think the logic is awesome as a first step without breaking changes. Thanks! 👍

[bastianallgeier]

I'm really glad we were able to finally merge this. What a marathon :) Thanks for your patience and your contribution, Roman!