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Hi Peter,
From the description provided, we totally understand your concern about adding the attachment for the SharePoint in the Planner task and changing the name of the attached file in the Document Library will give the 404 file not found when you access the attached file from the Planner.
We have tested this and found the same behavior at our end, the observation we find that URL of the file in the SharePoint will have the name of the file. So, when you attach the file in the Planner task, it will attach it as link where it contains the name of the file. And once you change the name of the file in the SharePoint Online Document Library, the URL of the file in the Document Library will be changed because the URL contains the name of the file.
So, this change doesn’t be reflected in the attachments added in the Planner where the URL will be the previous file name and opening this file will have 404 file not found error because there will be no any reference of this file in Document Library.
I do understand the inconvenience caused and apologies for it. Given the situation, I request you add your vote to related idea: Renaming a file in SharePoint breaks the file link in planner card - 404 in the Planner UserVoice as related team can take into consideration about it. Your feedback is highly appreciated as it helps to improve the functionality of the product.
Hope you understand and appreciate your patience. Have a good day and stay safe 😊
Best Regards,
Chitrahaas