Hi Marc,
As you know, when we choose Files to attach a file, these files are stored in the default document library associated with your plan's Microsoft 365 Group. If we move the attachments to a folder to organize them, we won't find the attachment when we click it in the Planner task and unfortunately there’s no option/setting to change the default save location.
I understand your requirement and I think it’s a good idea and in this regard, I would like to invite you to vote and share your feedback in Microsoft Planner UserVoice where other customers have requested the same feature. UserVoice is the best way to give the feedback to Product Group as it’s actively monitored by them and many new features have been implemented from our customer’s feedback.
Besides, though we can’t change the default location, there’s a workaround that you can consider, you may create a folder and upload the files to it in the document library first, then click Add attachment>SharePoint to select the files in your document library.
We appreciate your understanding. Have a nice day and stay safe 😊
Kind Regards,
Neha