composer: atlas export to image does not remember file format

[qgib]

Author Name: Mathieu Pellerin - nIRV (Mathieu Pellerin - nIRV)
Original Redmine Issue: 15186
Affected QGIS version: master
Redmine category:map_composer/printing
Assignee: Mathieu Pellerin - nIRV

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Contrary to a normal composer export to image, the atlas' export to image directory picker dialog doesn't remember the file format chosen by the user. While it's not the end of the world, it'd improve the workflow for that picker dialog to behave like other file pickers and remember the format.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• easy_fix was configured as 0
• regression was configured as 0

[qgib]

Author Name: Mathieu Pellerin - nIRV (Mathieu Pellerin - nIRV)

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• resolution was changed from to fixed/implemented
• assigned_to_id was changed from Nyall Dawson to Mathieu Pellerin - nIRV
• status_id was changed from Open to Closed