It seems Gnome 3 removed the button allowing users to add specified applications into the ‘Other Applications’ list under ‘Open With’ in file properties. Until the functionality is restored, you can add applications manually by doing the below:

cp /usr/share/applications/gedit.desktop ~/.local/share/applications/your_app.desktop

Modify the contents of your_app.deskop to look something like the below:

[Desktop Entry]
Name=your_app
GenericName=Your App
Comment=Edit text files
Keywords=Plaintext;Write;
Exec=your_app %U
Terminal=false
Type=Application
StartupNotify=true
MimeType=text/plain;
Icon=/path/to/icon.png
Categories=GNOME;GTK;Utility;TextEditor;
Actions=Window;Document;
X-Ubuntu-Gettext-Domain=your_app

You can copy any .desktop file from /usr/share/applications so pick the one that closest resembles the application you’re adding. Below you can see komodo added to mine: