Existe-t-il un moyen de définir une légende par défaut pour toutes les images téléchargées

Iln'y a vraimentpasencore de documentation à ce sujet,mais vouspourrezprobablement lefaireen vous connectant aufiltre attachment_fields_to_saveet en yinsérant la légendepar défaut.

Depuis le Codex :

attachment_fields_to_save
appliqué aux champs associés à un piècejointe avant de lesenregistrer dans labase de données. Appelé dans le fonctionmedia_upload_form_handler. Arguments de lafonction defiltrage: an tableau d'attributs depublication,untableau de champs depiècejointe comprenant le modifications soumises àpartir duformulaire

Ilest défini sur wp- admin/includes/media.php :

 // TESTED :)
function wpse300512_image_attachment_fields_to_save($post, $attachment) {
if ( substr($post['post_mime_type'], 0, 5) == 'image' ) {
    if ( '' === trim( $post['post_title'] ) ) {
        $post['post_title'] = preg_replace('/\.\w+$/', '', basename($post['guid']));
        $post['errors']['post_title']['errors'][] = __('Empty Title filled from filename.');
    }
    // captions are saved as the post_excerpt, so we check for it before overwriting
    if ( '' === trim( $post['post_excerpt'] ) ) {
        $post['post_excerpt'] = 'default caption';
    }
}

return $post;
}

add_filter('attachment_fields_to_save', 'wpse300512_image_attachment_fields_to_save', 10, 2);
 

MISE À JOUR: J'ai réussi à letester,et celafonctionnetel quel. Déposez-le simplement sur votrefunctions.php :)

There's really no documentation for it yet, but you'll probably be able to do it hooking to the attachment_fields_to_save filter and inserting the default caption there.

From the Codex:

attachment_fields_to_save
applied to fields associated with an attachment prior to saving them in the database. Called in the media_upload_form_handler function. Filter function arguments: an array of post attributes, an array of attachment fields including the changes submitted from the form

It is defined on wp-admin/includes/media.php:

// TESTED :)
function wpse300512_image_attachment_fields_to_save($post, $attachment) {
if ( substr($post['post_mime_type'], 0, 5) == 'image' ) {
    if ( '' === trim( $post['post_title'] ) ) {
        $post['post_title'] = preg_replace('/\.\w+$/', '', basename($post['guid']));
        $post['errors']['post_title']['errors'][] = __('Empty Title filled from filename.');
    }
    // captions are saved as the post_excerpt, so we check for it before overwriting
    if ( '' === trim( $post['post_excerpt'] ) ) {
        $post['post_excerpt'] = 'default caption';
    }
}

return $post;
}

add_filter('attachment_fields_to_save', 'wpse300512_image_attachment_fields_to_save', 10, 2);

UPDATE: I managed to test it, and it works as is. Just drop it on your functions.php :)