File association is used to determine what program to use to open a file. All the file associations for Ranger can be found in
~/.config/ranger/rifle.conf file. This file is well documented and should answer most of your queries. Note that the file is read from top to bottom and the rule that matches first is chosen and applied.
In Ranger, you can choose a program to open a file based on its extension. For example, to open a JPEG file with Feh image viewer, add this line at the top of
ext jpg = feh "$@"
After the specific file associations, I like to have a rule to catch all the rest of the files and open them using
has xdg-open, flag f = xdg-open "$1"
For more info on configuring Ranger, refer to the excellent documentation by Arch here.
Tried with: Ranger 1.6.1 and Ubuntu 14.04