The figure, Viewing a MIME-encoded message, shows how a mail message that contains MIME information appears in the Reading window.
Incoming mail encoded with MIME information is identified as such in the message list in the Folder contents viewing area of the Main window. The letter "M" is displayed to the right of the message number. Also, if icons are enabled for the message list, the icon will be colored with a rainbow hue versus plain white.
When a MIME message is selected for reading the Reading window is presented. Message body parts containing plain ASCII text or Rich Text are displayed directly in the Reading window. Rich Text attributes, such as font changes, highlighting, centering/justification/indentation, and so forth, are used to modify the presentation of the text in the window.
Message body parts containing other data types are represented in the Reading window with a box containing two buttons and the description tag for the body part. The buttons are labeled Show and Hide. Pressing the Show button causes the appropriate viewer program to be launched to display the body part. Pressing the Hide button cancels the viewer.
There are two kinds of body parts, in-line and external. One way to think of these is "copy by value" and "copy by reference," respectively. In the preceding figure, the second box shows an external body part. It's a GIF file which can be accessed as a local file (actually, it's a remote file accessed through NFS, but that's transparent to the user), and the file's path name is given. Pressing the Show button will launch a GIF viewer to display this file.