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Dear respected A Hairy Good Man,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, I'm sorry and I'm afraid to say that currently unfortunately all attachments in a task in planner goes to its related SharePoint site document library.
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.
We feel your pain due to this limitation and we’re really feel sorry for it. However, we appreciate your understanding. I do understand the inconvenience and apologize for it.
Personally, I completely agree with you, this is indeed a good addition to the current features of Microsoft Office 365 application. We sincerely recommend you, regarding to your concern, I request you, please send your feedback and write down what you want them to implement via this link: Microsoft Planner feedback portal, Microsoft product developers team monitor this site 24/7 and they will receive your request and put it into consideration. This is the best platform to let related team hear from you and make the products and services better for you and others.
We can expect this feature to be used in the future.
I'm really sorry I deliver you bad news and I apologize for it.
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.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Waqas Muhammad