diff --git a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/watcher/support/WatcherIndexTemplateRegistryField.java b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/watcher/support/WatcherIndexTemplateRegistryField.java
index 4cf0898bae2..25e2c928d9a 100644
--- a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/watcher/support/WatcherIndexTemplateRegistryField.java
+++ b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/watcher/support/WatcherIndexTemplateRegistryField.java
@@ -12,8 +12,9 @@ public final class WatcherIndexTemplateRegistryField {
     // version 3: include watch status in history
     // version 6: upgrade to ES 6, removal of _status field
     // version 7: add full exception stack traces for better debugging
+    // version 8: fix slack attachment property not to be dynamic, causing field type issues
     // Note: if you change this, also inform the kibana team around the watcher-ui
-    public static final String INDEX_TEMPLATE_VERSION = "7";
+    public static final String INDEX_TEMPLATE_VERSION = "8";
     public static final String HISTORY_TEMPLATE_NAME = ".watch-history-" + INDEX_TEMPLATE_VERSION;
     public static final String TRIGGERED_TEMPLATE_NAME = ".triggered_watches";
     public static final String WATCHES_TEMPLATE_NAME = ".watches";
diff --git a/x-pack/plugin/core/src/main/resources/watch-history.json b/x-pack/plugin/core/src/main/resources/watch-history.json
index a26305b3554..d158281c264 100644
--- a/x-pack/plugin/core/src/main/resources/watch-history.json
+++ b/x-pack/plugin/core/src/main/resources/watch-history.json
@@ -507,6 +507,13 @@
                               "properties" : {
                                 "color" : {
                                   "type" : "keyword"
+                                },
+                                "fields" : {
+                                  "properties" : {
+                                    "value" : {
+                                      "type" : "text"
+                                    }
+                                  }
                                 }
                               }
                             }
diff --git a/x-pack/qa/third-party/slack/src/test/resources/rest-api-spec/test/slack/10_slack.yml b/x-pack/qa/third-party/slack/src/test/resources/rest-api-spec/test/slack/10_slack.yml
index 3b04ba71675..259bc9e1d25 100644
--- a/x-pack/qa/third-party/slack/src/test/resources/rest-api-spec/test/slack/10_slack.yml
+++ b/x-pack/qa/third-party/slack/src/test/resources/rest-api-spec/test/slack/10_slack.yml
@@ -16,7 +16,13 @@
             },
             "input": {
               "simple": {
-                "foo" : "something from input"
+                "foo" : "something from input",
+                "hits" : {
+                  "hits" : [
+                    { "_source" : { "name" : "first",  "value" : "2018-04-26T11:45:12.518Z" } },
+                    { "_source" : { "name" : "second", "value" : "anything" } }
+                  ]
+                }
               }
             },
             "actions": {
@@ -49,7 +55,20 @@
                           }
                         ]
                       }
-                    ]
+                    ],
+                    "dynamic_attachments" : {
+                      "list_path" : "ctx.payload.hits.hits",
+                      "attachment_template" : {
+                        "title": "Title",
+                        "fields" : [
+                          {
+                            "title" : "Field title {{_source.name}}",
+                            "value" : "{{_source.value}}",
+                            "short" : true
+                          }
+                        ]
+                      }
+                    }
                   }
                 }
               }