Post a message whose attachment can't be previewed instead of failing

The thumbnail or video preview is generated while the message is posted.
When ffmpeg or libvips can't decode the file (a truncated upload, an .mp4
holding only audio), the error escaped after the message had been saved:
the request failed with a 500, the message was never broadcast, and the
sender's upload stayed at 100% while the rest of the files in that drop
were never sent. Posting the message without a preview keeps the file and
lets everyone see it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JVFo3Lt9T8M5NR7KxVvsZ2

app/models/message/attachment.rb

@@ -30,6 +30,7 @@ module Message::Attachment
       attachment&.analyze
     end
 
+    # A file that ffmpeg or libvips can't decode is still the message: post it without a preview.
     def process_attachment_thumbnail
       case
       when attachment.video?
@@ -37,5 +38,7 @@ module Message::Attachment
       when attachment.representable?
         attachment.representation(:thumb).processed
       end
+    rescue ActiveStorage::PreviewError, Vips::Error => error
+      Rails.logger.warn "Posted #{attachment.filename} without a preview: #{error.class}: #{error.message.lines.first&.chomp}"
     end
 end