diff --git a/CRM/Core/BAO/CustomField.php b/CRM/Core/BAO/CustomField.php
index 5c49f22862c3..5e59d3c9b2f4 100644
--- a/CRM/Core/BAO/CustomField.php
+++ b/CRM/Core/BAO/CustomField.php
@@ -2010,6 +2010,68 @@ public static function moveField($fieldID, $newGroupID) {
     CRM_Utils_System::flushCache();
   }
 
+  /**
+   * Move custom data from one contact to another.
+   *
+   * This is currently the start of a refactoring. The theory is each
+   * entity could have a 'move' function with a DAO default one to fall back on.
+   *
+   * At the moment this only does a small part of the process - ie deleting a file field that
+   * is about to be overwritten. However, the goal is the whole process around the move for
+   * custom data should be in here.
+   *
+   * This is currently called by the merge class but it makes sense that api could
+   * expose move actions as moving (e.g) contributions feels like a common
+   * ask that should be handled by the form layer.
+   *
+   * @param int $oldContactID
+   * @param int $newContactID
+   * @param int[] $fieldIDs
+   *   Optional list field ids to move.
+   *
+   * @throws \CiviCRM_API3_Exception
+   * @throws \Exception
+   */
+  public function move($oldContactID, $newContactID, $fieldIDs) {
+    if (empty($fieldIDs)) {
+      return;
+    }
+    $fields = civicrm_api3('CustomField', 'get', ['id' => ['IN' => $fieldIDs], 'return' => ['custom_group_id.is_multiple', 'custom_group_id.table_name', 'column_name', 'data_type'], 'options' => ['limit' => 0]])['values'];
+    $return = [];
+    foreach ($fieldIDs as $fieldID) {
+      $return[] = 'custom_' . $fieldID;
+    }
+    $oldContact = civicrm_api3('Contact', 'getsingle', ['id' => $oldContactID, 'return' => $return]);
+    $newContact = civicrm_api3('Contact', 'getsingle', ['id' => $newContactID, 'return' => $return]);
+
+    // The moveaAllBelongings function has functionality to move custom fields. It doesn't work very well...
+    // @todo handle all fields here but more immediately Country since that is broken at the moment.
+    $fieldTypesNotHandledInMergeAttempt = ['File'];
+    foreach ($fields as $field) {
+      $isMultiple = !empty($field['custom_group_id.is_multiple']);
+      if ($field['data_type'] === 'File' && !$isMultiple) {
+        if (!empty($oldContact['custom_' . $field['id']]) && !empty($newContact['custom_' . $field['id']])) {
+          CRM_Core_BAO_File::deleteFileReferences($oldContact['custom_' . $field['id']], $oldContactID, $field['id']);
+        }
+        if (!empty($oldContact['custom_' . $field['id']])) {
+          CRM_Core_DAO::executeQuery("
+            UPDATE civicrm_entity_file
+            SET entity_id = $newContactID
+            WHERE file_id = {$oldContact['custom_' . $field['id']]}"
+          );
+        }
+      }
+      if (in_array($field['data_type'], $fieldTypesNotHandledInMergeAttempt) && !$isMultiple) {
+        CRM_Core_DAO::executeQuery(
+          "INSERT INTO {$field['custom_group_id.table_name']} (entity_id, {$field['column_name']})
+          VALUES ($newContactID, {$oldContact['custom_' . $field['id']]})
+          ON DUPLICATE KEY UPDATE
+          {$field['column_name']} = {$oldContact['custom_' . $field['id']]}
+        ");
+      }
+    }
+  }
+
   /**
    * Get the database table name and column name for a custom field.
    *
@@ -2020,6 +2082,7 @@ public static function moveField($fieldID, $newGroupID) {
    *
    * @return array
    *   fatal is fieldID does not exists, else array of tableName, columnName
+   * @throws \Exception
    */
   public static function getTableColumnGroup($fieldID, $force = FALSE) {
     $cacheKey = "CRM_Core_DAO_CustomField_CustomGroup_TableColumn_{$fieldID}";
diff --git a/CRM/Dedupe/Merger.php b/CRM/Dedupe/Merger.php
index 26b854fad6ed..1578acce6e2a 100644
--- a/CRM/Dedupe/Merger.php
+++ b/CRM/Dedupe/Merger.php
@@ -1512,12 +1512,13 @@ public static function getRowsElementsAndInfo($mainId, $otherId, $checkPermissio
    * @param int $otherId
    *   Duplicate contact which would be deleted after merge operation.
    *
-   * @param $migrationInfo
+   * @param array $migrationInfo
    *
    * @param bool $checkPermissions
    *   Respect logged in user permissions.
    *
    * @return bool
+   * @throws \CiviCRM_API3_Exception
    */
   public static function moveAllBelongings($mainId, $otherId, $migrationInfo, $checkPermissions = TRUE) {
     if (empty($migrationInfo)) {
@@ -1558,6 +1559,10 @@ public static function moveAllBelongings($mainId, $otherId, $migrationInfo, $che
 
     // **** Do contact related migrations
     $customTablesToCopyValues = self::getAffectedCustomTables($submittedCustomFields);
+    // @todo - move all custom field processing to the move class & eventually have an
+    // overridable DAO class for it.
+    $customFieldBAO = new CRM_Core_BAO_CustomField();
+    $customFieldBAO->move($otherId, $mainId, $submittedCustomFields);
     CRM_Dedupe_Merger::moveContactBelongings($mainId, $otherId, $moveTables, $tableOperations, $customTablesToCopyValues);
     unset($moveTables, $tableOperations);
 
@@ -1601,13 +1606,10 @@ public static function moveAllBelongings($mainId, $otherId, $migrationInfo, $che
     if (!isset($submitted)) {
       $submitted = [];
     }
-    $customFiles = [];
     foreach ($submitted as $key => $value) {
-      list($cFields, $customFiles, $submitted) = self::processCustomFields($mainId, $key, $cFields, $customFiles, $submitted, $value);
+      list($cFields, $submitted) = self::processCustomFields($mainId, $key, $cFields, $submitted, $value);
     }
 
-    self::processCustomFieldFiles($mainId, $otherId, $customFiles);
-
     // move view only custom fields CRM-5362
     $viewOnlyCustomFields = [];
     foreach ($submitted as $key => $value) {
@@ -2146,29 +2148,36 @@ protected static function swapOutFieldsAffectedByQFZeroBug(&$migrationInfo) {
   /**
    * Honestly - what DOES this do - hopefully some refactoring will reveal it's purpose.
    *
+   * Update this is formatting fields to be processed through 'ProfileContactCreate action
+   * - for some fields it fails - e.g Country - per testMergeCustomFields.
+   *
+   * Goal is to move all custom field handling into 'move' functions on the various BAO
+   * with an underlying DAO function. For custom fields it has been started on the BAO.
+   *
    * @param $mainId
    * @param $key
    * @param $cFields
-   * @param $customFiles
    * @param $submitted
    * @param $value
    *
    * @return array
+   * @throws \Exception
    */
-  protected static function processCustomFields($mainId, $key, $cFields, $customFiles, $submitted, $value) {
+  protected static function processCustomFields($mainId, $key, $cFields, $submitted, $value) {
     if (substr($key, 0, 7) == 'custom_') {
       $fid = (int) substr($key, 7);
       if (empty($cFields[$fid])) {
-        return [$cFields, $customFiles, $submitted];
+        return [$cFields, $submitted];
       }
       $htmlType = $cFields[$fid]['attributes']['html_type'];
       switch ($htmlType) {
         case 'File':
-          $customFiles[] = $fid;
+          // Handled in CustomField->move(). Tested in testMergeCustomFields.
           unset($submitted["custom_$fid"]);
           break;
 
         case 'Select Country':
+          // @todo Test in testMergeCustomFields disabled as this does not work, Handle in CustomField->move().
         case 'Select State/Province':
           $submitted[$key] = CRM_Core_BAO_CustomField::displayValue($value, $fid);
           break;
@@ -2247,7 +2256,7 @@ protected static function processCustomFields($mainId, $key, $cFields, $customFi
           break;
       }
     }
-    return [$cFields, $customFiles, $submitted];
+    return [$cFields, $submitted];
   }
 
   /**
@@ -2390,64 +2399,6 @@ public static function getConflicts(&$migrationInfo, $mainId, $otherId, $mode) {
     return $conflicts;
   }
 
-  /**
-   * Do file custom fields related migrations.
-   * FIXME: move this someplace else (one of the BAOs) after discussing
-   * where to, and whether CRM_Core_BAO_File::deleteFileReferences() shouldn't actually,
-   * like, delete a file...
-   *
-   * Note outstanding bug https://lab.civicrm.org/dev/core/issues/723
-   * relates to this code....
-   *
-   * @param $mainId
-   * @param $otherId
-   * @param $customFiles
-   */
-  protected static function processCustomFieldFiles($mainId, $otherId, $customFiles) {
-    foreach ($customFiles as $customId) {
-      list($tableName, $columnName, $groupID) = CRM_Core_BAO_CustomField::getTableColumnGroup($customId);
-
-      // get the contact_id -> file_id mapping
-      $fileIds = [];
-      $sql = "SELECT entity_id, {$columnName} AS file_id FROM {$tableName} WHERE entity_id IN ({$mainId}, {$otherId})";
-      $dao = CRM_Core_DAO::executeQuery($sql);
-      while ($dao->fetch()) {
-        // @todo - this is actually broken - fix & or remove - see testMergeCustomFields
-        $fileIds[$dao->entity_id] = $dao->file_id;
-        if ($dao->entity_id == $mainId) {
-          CRM_Core_BAO_File::deleteFileReferences($fileIds[$mainId], $mainId, $customId);
-        }
-      }
-
-      // move the other contact's file to main contact
-      //NYSS need to INSERT or UPDATE depending on whether main contact has an existing record
-      if (CRM_Core_DAO::singleValueQuery("SELECT id FROM {$tableName} WHERE entity_id = {$mainId}")) {
-        $sql = "UPDATE {$tableName} SET {$columnName} = {$fileIds[$otherId]} WHERE entity_id = {$mainId}";
-      }
-      else {
-        $sql = "INSERT INTO {$tableName} ( entity_id, {$columnName} ) VALUES ( {$mainId}, {$fileIds[$otherId]} )";
-      }
-      CRM_Core_DAO::executeQuery($sql);
-
-      if (CRM_Core_DAO::singleValueQuery("
-        SELECT id
-        FROM civicrm_entity_file
-        WHERE entity_table = '{$tableName}' AND file_id = {$fileIds[$otherId]}")
-      ) {
-        $sql = "
-          UPDATE civicrm_entity_file
-          SET entity_id = {$mainId}
-          WHERE entity_table = '{$tableName}' AND file_id = {$fileIds[$otherId]}";
-      }
-      else {
-        $sql = "
-          INSERT INTO civicrm_entity_file ( entity_table, entity_id, file_id )
-          VALUES ( '{$tableName}', {$mainId}, {$fileIds[$otherId]} )";
-      }
-      CRM_Core_DAO::executeQuery($sql);
-    }
-  }
-
   /**
    * @param $rule_group_id
    * @param $group_id
diff --git a/tests/phpunit/api/v3/ContactTest.php b/tests/phpunit/api/v3/ContactTest.php
index 4cab52bddcc7..1e26ecd9bd1b 100644
--- a/tests/phpunit/api/v3/ContactTest.php
+++ b/tests/phpunit/api/v3/ContactTest.php
@@ -3386,16 +3386,15 @@ public function testMerge() {
    */
   public function testMergeCustomFields() {
     $contact1 = $this->individualCreate();
-    /* Not sure this is quite right but it does get it into the file table
+    // Not sure this is quite right but it does get it into the file table
     $file = $this->callAPISuccess('Attachment', 'create', [
-    'name' => 'header.txt',
-    'mime_type' => 'text/plain',
-    'description' => 'My test description',
-    'content' => 'My test content',
-    'entity_table' => 'civicrm_contact',
-    'entity_id' => $contact1,
+      'name' => 'header.txt',
+      'mime_type' => 'text/plain',
+      'description' => 'My test description',
+      'content' => 'My test content',
+      'entity_table' => 'civicrm_contact',
+      'entity_id' => $contact1,
     ]);
-     */
 
     $this->createCustomGroupWithFieldsOfAllTypes();
     $fileField = $this->getCustomFieldName('file');
@@ -3407,7 +3406,7 @@ public function testMergeCustomFields() {
     $countriesByName = array_flip(CRM_Core_PseudoConstant::country(FALSE, FALSE));
     $customFieldValues = [
       // @todo fix the fatal bug on this & uncomment - see dev/core#723
-      // $fileField => $file['id'],
+      $fileField => $file['id'],
       $linkField => 'http://example.org',
       $dateField => '2018-01-01 17:10:56',
       $selectField => 'G',
@@ -3425,6 +3424,7 @@ public function testMergeCustomFields() {
       'auto_flip' => FALSE,
     ]);
     $contact = $this->callAPISuccessGetSingle('Contact', ['id' => $contact2, 'return' => array_keys($customFieldValues)]);
+    $this->assertEquals($contact2, CRM_Core_DAO::singleValueQuery('SELECT entity_id FROM civicrm_entity_file WHERE file_id = ' . $file['id']));
     foreach ($customFieldValues as $key => $value) {
       $this->assertEquals($value, $contact[$key]);
     }