some time your application may not added in open with list although it is install in your system.

for example I use idle to open python source file (uce.py). but it is not include in open with list or default app. 

to add any app as a default app or open with list :-

1) goto /usr/share/applications folder and see your app is there or not.

if not create launcher.                                    /////           more info see how to create luncher

      name: idle                                   /// need to cread a .desktop file

      command:  app-name

2) go to /home/user-name/.local/share/applications and open mimeapps.list

(gedit ~/.local/share/applications/mimeapps.list)

and add it  as

   [Default Applications]  // as a default app

or 

    [Added Associations]   // in the open with list

How to add in mimeapps.list

2) open mimeapps.list to edit

add a line as:

text/x-python=idle.desktop  //   type = create launcher name . desktop

3) save mimeapps.list 

you will see the effect.

thanks to read.