default script value (rebased onto develop)



This is the same as gh-2340 but rebased onto develop.

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To Test:
 * Log in.
 * Do not select any item. 
 * Click on the script button to display the available script.
 * Go to ```export scripts>Batch image export```
 * Check that The default data type is ```Image```. Close dialog
 * Repeat the previous steps and select dataset then an image then a project.

see gh-2306