Is there a way out in which we can change the mimetype of renditions matching to the original image's mimetype.
Example- Please find the below screenshot for more understanding.
The original image mimetype is of image/jpeg while the generated rendition's mimetype is image/png which I see has been mentioned as by default.
Try to update keepformat list in the dialog of update_asset with required mimetype.
If it doesn't work then you would have to customize it like -
Adobe Experience Manager Help | Creating a Custom Adobe Experience Manager Dam Media Handler