Plone 5 and upwards needs this index to figure out the icon.

The older getIcon Index return bool for either showing or not showing a thumbnail in folder_contents.

solr-base.cfg

@@ -14,7 +14,7 @@ parts =
 
 # Increment this value, if you change solr related configs in this file.
 # This makes the solr recipe detect a change and perform a reinstall.
-config-version = 4
+config-version = 5
 
 
 [solr-settings]
@@ -173,7 +173,7 @@ additional-solrconfig =
             ${:solrconfig-shards}
         </lst>
         <lst name="invariants">
-            <str name="fl">Creator Title Description modified portal_type path_string getIcon UID getRemoteUrl</str>
+            <str name="fl">Creator Title Description modified portal_type path_string getIcon UID getRemoteUrl mime_type</str>
         </lst>
         <arr name="last-components">
             <str>spellcheck</str>
@@ -187,7 +187,7 @@ additional-solrconfig =
             <int name="rows">1000</int>
         </lst>
         <lst name="invariants">
-            <str name="fl">Title Description portal_type path_string getIcon getRemoteUrl Type</str>
+            <str name="fl">Title Description portal_type path_string getIcon getRemoteUrl Type mime_type</str>
         </lst>
     </requestHandler>
 
@@ -224,3 +224,4 @@ index =
     name:UID                    type:string stored:true required:true
     name:crawler_site_id        type:string stored:true
     name:trashed                type:boolean stored:true
+    name:mime_type              type:string stored:true