message: fix devlog post text media item upload
diff: preview-url="/web/dash/developer/games/devlog/preview"
  markdown-mode="devlog"
  :show-media-items="true"
  - media-itemm-type="fireside-post-image"
  + media-item-type="fireside-post-image"
  :placeholder="$gettext( `Write your post content here...` )"
  />
  