[6.x] Allow a symfony file instance in validate dimensions

[tobiasthaden]

The Problem

When a user validates the dimensions of a Symfony\Component\HttpFoundation\File\File via the Illuminate/Validation/Concerns/ValidatesAttributes trait the request throws an error within validateDimensions because the getClientMimeType() method does not exist on the instance.

Only UploadedFile has the currently used getClientMimeType() method.

The Solution

This PR fixes that issue by using the getMimeType()method that exists on both valid file instances.

Additional

According to Symfony documentation:

The client mime type is extracted from the request from which the file
was uploaded, so it should not be considered as a safe value.

For a trusted mime type, use getMimeType() instead (which guesses the mime
type based on the file content).

What does this break?

The mime type will not be extracted from the request but guessed from the files content. So the uploaded files must be valid. In case of a .svg the file must contain the opening <xml> tag otherwise it will be interpreted as plain/text and the validation will fail.

I think that is not a breaking change, because uploaded files should – in the past also – always be valid.

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Resolves #29962