routing what looks like static files to a controller?

Hello,

I noticed that image_tag() likes to append an extension onto the src
path if it doesn't already have one. That's all fine and good, but how
do I get that resulting path to be routed to a controller/action?

For instance, the url:
/content/thumbnail/123.jpg
fails to get routed to :controller => 'content', :action => 'thumbnail'

How do I make it get routed to that controller/action?

Thanks for the help.