Added support for select multiple attribute in array filter

system/blueprints/pages/default.yaml

@@ -126,6 +126,7 @@ form:
                 header.taxonomy:
                   type: taxonomy
                   label: Taxonomy
+                  multiple: true
                   validate:
                     type: array
 

system/src/Grav/Common/Data/Validation.php

@@ -492,6 +492,7 @@ class Validation
     {
         $values = (array) $value;
         $options = isset($field['options']) ? array_keys($field['options']) : array();
+        $multi = isset($field['multiple']) ? $field['multiple'] : false;
 
         if ($options) {
             $useKey = isset($field['use']) && $field['use'] == 'keys';
@@ -500,6 +501,12 @@ class Validation
             }
         }
 
+        if ($multi) {
+            foreach ($values as $key => $value) {
+                $values[$key] = explode(',', $value[0]);
+            }
+        }
+
         return $values;
     }