How to Migrate Email Attachments from CRM On premise to D365 Online?

I am trying to migrate attachments from CRM 2011 on premise to D365 Online. The Email body and activity parties have migrated fine. But when I try to migrate the Attachments I get the following:

 

Operation failed. Label: Create activitymimeattachment, Name: activitymimeattachmentCreate, Message: 2:The following error has occurred: Could not find file 'C:\Windows\system32\abcd.pdf'. : Error in calling Operation Create The following error has occurred: System.IO.FileNotFoundException: 

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