Download image

Fix the download issue noticed while testing #1131. 
To test
 * select an MIF. 
 * Download.
 * A zip with all the images should be created 
 * Select a single image
 * Make sure it works too.

The commit from #1131 has been cherry-picked to avoid PR conflicts.

The commit  ``` 80f62b6 ``` was already in ```dev_4_4``` but not in ```develop```
The commit  ```52bb42a``` will have to be back ported to ```dev_4_4```