The reason this happens is due to the format of the message. In Rich Text format,(Supported since Outlook 97) it does in-line attachments in the body of the e-mail message as shown below.
Did you notice the attachment to the message sent in HTML format also has the size of the file (26 KB) while the message in Rich Text Format has the same attachment but no indication of file size?
So how do you change the format of the message from Rich Text to HTML? You can do it two ways. You can change it per message by opening a new message then going to FORMAT TEXT in Outlook and choosing HTML as shown below (applies to Office 2010 and Office 2013)
If you want to only send HTML Messages and have attachments be at the top of the message and not embedded in the body you can change it in the Outlook Client settings By going to FILE then OPTIONS then Mail and select HTML as shown below from Office 2103.
- Open the contact card for the recipient.
- In the E-mail box, double-click the recipient's e-mail address.
- In the Internet Format list, select the format that you want to use for messages to this recipient.