Dear Hasib Khwaja,
Thank you for reaching out to the Microsoft community. We are delighted to offer our assistance.
If a user has their desktop synced to OneDrive, when they attach a file from their desktop in Outlook, it will show as being attached from SharePoint. This is because OneDrive is a part of SharePoint and the file is synced from the user’s desktop to OneDrive and then attached from there. There is no direct way to stop this behavior, but the user can choose to attach the file as a local copy instead of from OneDrive. To do this, the user can select the “Attach File” option in Outlook, then choose “Browse This PC” and select the file from their local desktop. This will attach the file as a local copy and not show it as being attached from SharePoint.
Thank you for your understanding and cooperation! Have a great day.
Sincerely,
Sean | Microsoft Community Moderator