What are you talking about?
Most email attachments are graphics and are displayed in the email itself. Video or sound files will play in the email's window with a single click. Any document file will open the necessary viewer and/or editor with the document as the focussed window with a double click.
In Leopard, a single click will allow you a "quick look" of any attachment that is playable, displayable, or editable. Quick Look displays or plays the graphic, movie, MP3 file, or document without opening the creation app or displayer. For example, you can open a Word file to see what it is without opening Word... or even having Word. Same with a Excel file. With PDF files you can open them without Acrobat Reader and even flip through their pages... all started with a single click.
In earlier incarnations of OSX, a single click will display ALL graphic attachments in an eMail as a slide show.
You WANT to open the attachment in an application that didn't create it... such as a Photoshop graphic in Preview? Simple... drag the attachment onto the Preview app icon on the dock and drop it. Preview will open and display the graphic. Open a Word document in Pages? Same thing... drag to Dock Icon or Application folder Icon and drop it. The Word doc will open in Pages.
What are you doing that requires SEVEN clicks?
Let me ask you this, and you may not know. Mac used to have some pretty steep fees to get in the game to produce software for Mac. (Unlike IBMs PC open archetecture approach)