Content Disposition
Content disposition is one of the attributes of an e-mail attachment defined by standards. It tells your software to display a particular attachment either in place (inline content disposition) or as icon (attachment content disposition).
Although most software is standards-compliant in this regard, the standard Mac OS X Mail application does not respect the content disposition of attachments at all, nor does it set it correctly for your outgoing mail.
With Mail Attachments Iconizer you have the option to respect the content disposition in incoming mail and attachments in outgoing mail have correctly set content disposition, so that the recipient will see them in the way you choose.
You can fine-tune how Mail interacts with the content disposition of attachments in the Iconizer's preferences. Content disposition of an individual attachment can be changed by selecting the option View in Place or View as Icon from the attachment context menu (you can open the menu by control-clicking on the attachment).