Post replacements: fix replacement message

It's incorrect to say that a new image was used, since we also host
videos. See for example https://danbooru.donmai.us/posts/3114352

app/logical/upload_service/replacer.rb

@@ -11,7 +11,7 @@ class UploadService
     end
 
     def comment_replacement_message(post, replacement)
-      %("#{replacement.creator.name}":[/users/#{replacement.creator.id}] replaced this post with a new image:\n\n#{replacement_message(post, replacement)})
+      %("#{replacement.creator.name}":[/users/#{replacement.creator.id}] replaced this post with a new file:\n\n#{replacement_message(post, replacement)})
     end
 
     def replacement_message(post, replacement)