Set up gallery image caption field

site/blueprints/files/gallery_image.yml

@@ -2,6 +2,25 @@ title: Image
 accept:
     mime: image/jpeg, image/png
 fields:
+    caption:
+        label:
+            en: Caption
+            fr: Légende
+        type: writer
+        toolbar:
+            inline: false
+        nodes: false
+        marks:
+            - bold
+            - italic
+            - '|'
+            - link
+            - email
+            - '|'
+            - clear
+        help:
+            en: "Informative text displayed below the image."
+            fr: "Texte informatif affiché sous l'image."
     alt_text:
         label:
             en: Alternative text