Fixes #3277.

This PR is written on top of #3280 which provides a way to see if a key is missing, and regenerate the meta when needed.

Mimicking Laravel validation rules
The new asset validation rules in this PR mimic the behaviour of the original Laravel rules. The Laravel code for all this magic can be found in Illuminate\Validation\Concerns\ValidatesAttributes.

Laravel actually has two validation rules to check a file's mime type. One is called mimes and, contrary to what you might expect, you should provide a list of extensions rather than mime types, like so: mimes:jpg,png. It doesn't use the actual file extension, instead it guesses the extension based on the mime type of the file.

People found it confusing that you had to provide a list of extensions to this rule. So later a second rule was added called mimetypes. This rule actually takes a list of mime types, e.g. mimetypes:image/jpg,image/png.

Using the guessed extension elsewhere too
I wonder, should the Asset class use the guessed extension too now for all the extension-based tests like isImage() and isVideo()?

Let's say you rename your image.jpg to image.rad and you upload it to an asset field that expects an image. Meaning it has an image validator or something like mimes:jpg,png. The new validators will be perfectly fine with your renamed file, because the guessed extension will still be jpg. But Asset.isImage() would currently return false, because it looks at the actual file extension, which is now rad. This is inconsistent behaviour.