Add custom upload validator to allow_upload/3 options

[arthurnovak]

Currently, allow_upload/3 supports validation only for entry size and accepted file types via the :max_file_size and :accept options. It would be useful to allow custom validation logic to run against each upload entry and return tailored errors when validation fails.

This change introduces support for custom upload validation by adding a :validator option to allow_upload/3.

The validator is a user-defined function that is executed for each upload entry and can return custom error reasons when validation does not pass.

:validator - An optional 1-arity function for performing custom validation on each upload entry. The function receives the upload entry and must return either :ok or {:error, reason}.

When an error tuple is returned, the entry is marked as failed and the error is exposed as reason via upload_errors/2.

@preciz @SteffenDE

[andrewtimberlake]

This would be very useful. I’ve recently jumped through some hoops to validate that uploaded images are square cropped for example.

[arthurnovak]

hey @preciz @SteffenDE, any chance you can have a look at it?

[preciz]

I can but I'm not a maintainer, so let's wait for them as usual.