Add a validation function for Dropzone before accepting files

[shaadcode]

Currently, there is no documented way to perform direct file validation before the Dropzone component accepts files.

It would be very useful to have a built-in validation function that runs before files are dropped and determines whether they should be accepted or rejected.

Problem

While it’s technically possible to handle file validation manually (e.g., by filtering in onDrop), this approach doesn’t integrate well with the Dropzone.Reject component — since that component is handled internally, likely via the accept prop.

As a result, the current behavior makes it hard to show the appropriate visual feedback (using Dropzone.Accept and Dropzone.Reject) for custom validation logic.

Proposal

Introduce a validate (or validator) function prop for <Dropzone>, for example:

<Dropzone
  validate={(files) => {
    // Return true if valid, false or a message if not
    return files.every(file => file.size < 5 * 1024 * 1024);
  }}
>
  <Dropzone.Accept>✅ Valid file</Dropzone.Accept>
  <Dropzone.Reject>❌ File too large</Dropzone.Reject>
</Dropzone>

If validate returns false or throws an error, the Dropzone.Reject component should be displayed automatically.