txt thumbnail definition for txt missing

We have a bunch of 500s in production similar to this one:

Processing by AssetsController#show as TEXT
  Parameters: {"id"=>"363954", "path"=>"filename_txt"}
Completed 500 Internal Server Error in 75ms (ActiveRecord: 61.4ms)

RuntimeError (No thumbnail definition found for txt 2544517):
  app/models/thumbnail.rb:123:in `thumbdef'
  app/models/thumbnail.rb:146:in `proxy?'
  app/models/thumbnail.rb:59:in `generate'
  app/controllers/assets_controller.rb:56:in `file_to_send'
  app/controllers/assets_controller.rb:11:in `show'