Fix ZIP file path creation code (see #10804) (rebased onto develop)



This is the same as gh-1106 but rebased onto develop.

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To test, try downloading an image from a multi-file image set (like LEI). The zip file should save fine in any target folder. Check that it works with your home directory (i.e. try saving to ```/Users/<username>/```). Any other file export options from Insight should be checked too.
/cc @jburel