OS X automatically displays small attachments in-line and reserves the attachment icon for larger attachments, and while the View As Icon mechanism helps, but it's not a particularly "sticky" setting; AFAIK, there's not a good way to always disable that from happening.
In the same control-click'd popup menu as the View as Icon stuff mentioned by VikingOSX, you can select what tool you want to open the attachment, and can work with it no differently than when there's just the "attachment" icon present.
This is a long-standing design flaw in Apple Mail.
Prior to Mavericks, this command in Terminal would stop the annoying behavior:
- defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes
This Terminal command stopped working in Mail 7.1 under Mavericks. I have reported this to Apple as both a feature request and a bug. You should too.