Debian: HOWTO find a package containing a certain file

(this example works on Debian-based systems too):

# apt-file update && apt-file search /usr/share/doc/rails/README.railties
rails: usr/share/doc/rails/README.railties
#

This example was me searching a package containing /usr/share/doc/rails/README.railties file. The result tells that rails is the package.