Attachments are shown in a panel at the bottom of the message's body. Attachments of type 'text/plain' with no filename are directly shown in the body as well as in the list.

Attachments panel
Select the files to be saved (multi-selection is possible) in the attachments list and use the command. If several attachments are selected, a directory selector will appear. If only one attachment is selected, a filename and path selector will appear, so that it's possible to change the name of the file being saved. For each attachment with no predefined filename, the application will prompt for a filename before saving.
MIME types can be associated with an external program to be viewed. This is done in the database with the programs table. Currently, there's no provision in the user interface to view or edit the associations between mime types and programs, so it has to be done at the database level.