The file you're looking to edit is ~/.config/Thunar/uca.xml and the content you want to add at the bottom above the last line </actions> is below in the code box. Simply copy and paste the content from the code section directly below this line into your uca.xml, save and exit all instances of Thunar and on re-opening, the right click context menu you added will be available when right clicking on an ISO file.
----- EDIT -----
↓ This one ↓ will mount an ISO file. This will create a new folder with the same name as the ISO and a .mount appended to the folder name, in the same directory as the ISO file. The mounted contents of the ISO will appear in the folder and you can peruse them as you wish.
Code: Select all
<action>
<icon>preferences-devices-drive-optical-check</icon>
<name>Mount ISO</name>
<submenu></submenu>
<unique-id>1698628158097339-1</unique-id>
<command>mkdir %f.mount & fuseiso %f %f.mount</command>
<description></description>
<range>*</range>
<patterns>*.iso;*.ISO</patterns>
<other-files/>
</action>
So if you like what you see and are happy to use it.
1) Copy and paste operation into your uca.xml file
2) Close and restart Thunar File Manager
3) Have fun. Once the action has been added, if you want a different icon to the one I selected, or it has no icon because you removed a theme, open Thunars Edit menu → Configure Custom Actions → scroll down and select Mount ISO then click the gear icon to edit it. In the edit window, click the button next to Icon: and search for something that makes more sense to you. Having done this you'll have learned how simple it is to make a custom action, and you'll know where the config file for it is

----- EDIT 2 -----
Adjusted the code to provide better workflow and removed the cleanup as it was not reliable.
----- EDIT 3 -----
Massive cleanup, removed superfluous content, fall back to Thunar Device Management and added an icon.