posts: fix placeholder thumbnail for Flash files.

* Replace the "Download" placeholder thumbnail for Flash files with a
  new placeholder that specifically says it's a Flash file.
* Fix a bug where the Flash placeholder thumbnail was too small when
  using larger thumbnail sizes.
* Fix it so that media assets don't falsely consider Flash files to have
  thumbnails. This could potentially cause errors if someone tried to
  expunge, replace, or regenerate a Flash post.

app/models/post.rb

@@ -162,6 +162,8 @@ class Post < ApplicationRecord
     end
 
     def preview_file_url
+      # XXX hack to return placeholder thumbnail for Flash files the /posts.json API.
+      return Danbooru.config.storage_manager.file_url("/images/flash-preview.png") if media_asset.is_flash?
       media_asset.variant(:preview).file_url
     end