support email content-type

model/alerting/webhook.go

@@ -20,4 +20,5 @@ type Email struct {
 	} `json:"recipients" elastic_mapping:"recipients:{type:object}"`
 	Subject string `json:"subject" elastic_mapping:"subject:{type:text}"`
 	Body string `json:"body" elastic_mapping:"body:{type:text}"`
+	ContentType string `json:"content_type" elastic_mapping:"content_type:{type:keyword}"`
 }

service/alerting/action/email.go

@@ -27,6 +27,7 @@ func (act *EmailAction) Execute()([]byte, error){
 		"variables": util.MapStr{
 			"subject": act.Subject,
 			"body": act.Body,
+			"content_type": act.Data.ContentType,
 		},
 	}
 	emailMsgBytes := util.MustToJSONBytes(emailMsg)