When you run the Search-Mailbox cmdlet together with the Attachment property keyword in Exchange Server 2013, the search results list all items that contain the "attachment" query string.
This issue occurs because the Attachment property is not restricted to only search attachments in email messages.
To fix this issue, install Cumulative Update 12 for Exchange Server 2013.
After you install this cumulative update, you can use the AttachmentNames property keyword to limit the search scope to only the items that have the specified attachment file name. For example, you can use the AttachmentNames property keyword as in the following example:
Search-Mailbox user_name -EstimateResultOnly -SearchQuery AttachmentNames:attachment_name
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Learn about the terminology that Microsoft uses to describe software updates.