Hello,
To list the files of a package, run as root user:
then select "View" in the menu and finally select the package.
To find the package containing a file, I'm only aware of slackpkg.
Code:
slackpkg search filename
For the slackpkg search to work, you need to use slackpkg (at least use the command "slackpkg update" to initialize/update its local database). For further details on slackpkg, please refer to its manpages.