[11.x] Add support for mime types in Resend mail transport

[jayanratna]

This PR adds support for setting mime types on attachments inside your Resend Mailable class. With the introduction of the content_type parameter in the Resend email API, it is now possible to pass that information along to the Resend API.

With this addition it is now possible to use the withMime method to set a custom type for your attachment:

public function attachments(): array
{
    return [
        Attachment::fromPath('/path/to/file')
                ->as('name.pdf')
                ->withMime('application/pdf'),
    ];
}