I'll second that amendment.
after further review, this action can be modified, by following these steps from the Thunar website:
To add actions associated with a file type, perform the following steps:
In the main area, select a file of the type to which you want to add an action. (ie. open the folder: pictures, then right-click on any saved image.)
Choose File → Open With Other Application... from the main menu.
Either choose an application in the Open With dialog or select Use a custom command and browse to the program with which you wish to open this type.
The action you have chosen is now added to the list of actions for that particular file type. If you enabled the Use as default for this kind of file option or there was no prior action associated with the type, the newly added action is the default.
You may also add actions using the Open With button under File → Properties...