As each message comes in to Office 365, it is scanned for dangerous attachments. The message body can be read while that scan is in progress, but the attachments cannot because they aren't yet thought to be safe. While the attachment scan is ongoing–typically less than two minutes–you'll see a placeholder in the attachments area. Close the re-open the message to see if the scan is complete and the attachment is available. Note that even if you read the body of the email before the scan completed, following the scan, the message status will reset to unread.