Symptoms
After you install one of the following Microsoft SharePoint Server updates, you can no longer delete list item attachments in the item detail panel:
Cause
This issue occurs because of changes that were made in how a lookup item is displayed in the item detail panel.
Workaround
To delete list item attachments, follow these steps:
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At the left-bottom corner of the list page, select Return to classic SharePoint.
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Select the item from which you want to delete the attachments.
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Select Edit Item.
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Next to each attachment that you want to delete, select Delete, and then save the page.
