Add option to always show mimetype in viewer

A user may want to be able to see what mimetype they are viewing, so
that they can determine what program it may be opened in or for some
other reason.

The config option is under the [viewer] section and is called
'always-show-mime'. It defaults to false to preserve the current
behaviour.

config/aerc.conf.in

@@ -69,6 +69,11 @@ show-headers=false
 # Default: From|To,Cc|Bcc,Date,Subject
 header-layout=From|To,Cc|Bcc,Date,Subject
 
+# Whether to always show the mimetype of an email, even when it is just a single part
+#
+# Default: false
+always-show-mime=false
+
 [compose]
 #
 # Specifies the command to run the editor with. It will be shown in an embedded

config/config.go

@@ -77,10 +77,11 @@ type FilterConfig struct {
 }
 
 type ViewerConfig struct {
-	Pager        string
-	Alternatives []string
-	ShowHeaders  bool       `ini:"show-headers"`
-	HeaderLayout [][]string `ini:"-"`
+	Pager          string
+	Alternatives   []string
+	ShowHeaders    bool `ini:"show-headers"`
+	AlwaysShowMime bool `ini:"always-show-mime"`
+	HeaderLayout   [][]string `ini:"-"`
 }
 
 type AercConfig struct {

doc/aerc-config.5.scd

@@ -133,6 +133,11 @@ These options are configured in the *[viewer]* section of aerc.conf.
 
 	Default: false
 
+*always-show-mime*
+	Whether to always show the mimetype of an email, even when it is just a single part.
+
+	Default: false
+
 ## COMPOSE
 
 These options are configured in the *[compose]* section of aerc.conf.

widgets/msgviewer.go

@@ -38,9 +38,10 @@ type MessageViewer struct {
 
 type PartSwitcher struct {
 	ui.Invalidatable
-	parts       []*PartViewer
-	selected    int
-	showHeaders bool
+	parts          []*PartViewer
+	selected       int
+	showHeaders    bool
+	alwaysShowMime bool
 }
 
 func NewMessageViewer(acct *AccountView, conf *config.AercConfig,
@@ -165,6 +166,7 @@ func createSwitcher(switcher *PartSwitcher, conf *config.AercConfig,
 	store *lib.MessageStore, msg *models.MessageInfo) error {
 	var err error
 	switcher.showHeaders = conf.Viewer.ShowHeaders
+	switcher.alwaysShowMime = conf.Viewer.AlwaysShowMime
 
 	if len(msg.BodyStructure.Parts) == 0 {
 		switcher.selected = 0
@@ -312,7 +314,7 @@ func (ps *PartSwitcher) Event(event tcell.Event) bool {
 
 func (ps *PartSwitcher) Draw(ctx *ui.Context) {
 	height := len(ps.parts)
-	if height == 1 {
+	if height == 1 && !ps.alwaysShowMime {
 		ps.parts[ps.selected].Draw(ctx)
 		return
 	}