diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index 3b9f6d8c992..d70037f0d16 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -2857,5 +2857,20 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2017042600.01);
     }
 
+    if ($oldversion < 2017050300.01) {
+        // MDL-58684:
+        // Remove all portfolio_tempdata records as these may contain serialized \file_system type objects, which are now unable to
+        // be unserialized because of changes to the file storage API made in MDL-46375. Portfolio now stores an id reference to
+        // files instead of the object.
+        // These records are normally removed after a successful export, however, can be left behind if the user abandons the
+        // export attempt (a stale record). Additionally, each stale record cannot be reused and is normally cleaned up by the cron
+        // task core\task\portfolio_cron_task. Since the cron task tries to unserialize them, and generates a warning, we'll remove
+        // all records here.
+        $DB->delete_records_select('portfolio_tempdata', 'id > ?', [0]);
+
+        // Main savepoint reached.
+        upgrade_main_savepoint(true, 2017050300.01);
+    }
+
     return true;
 }
diff --git a/lib/portfolio/plugin.php b/lib/portfolio/plugin.php
index 34c40c8d3fd..18572f1e98e 100644
--- a/lib/portfolio/plugin.php
+++ b/lib/portfolio/plugin.php
@@ -637,6 +637,11 @@ abstract class portfolio_plugin_base {
      * @return array|string|int|boolean value of the field
      */
     public final function get($field) {
+        // This is a legacy change to the way files are get/set.
+        // We now only set $this->file to the id of the \stored_file. So, we need to convert that id back to a \stored_file here.
+        if ($field === 'file') {
+            return $this->get_file();
+        }
         if (property_exists($this, $field)) {
             return $this->{$field};
         }
@@ -654,6 +659,12 @@ abstract class portfolio_plugin_base {
      * @return bool
      */
     public final function set($field, $value) {
+        // This is a legacy change to the way files are get/set.
+        // Make sure we never save the \stored_file object. Instead, use the id from $file->get_id() - set_file() does this for us.
+        if ($field === 'file') {
+            $this->set_file($value);
+            return true;
+        }
         if (property_exists($this, $field)) {
             $this->{$field} =& $value;
             $this->dirty = true;
@@ -776,7 +787,7 @@ abstract class portfolio_plugin_push_base extends portfolio_plugin_base {
  */
 abstract class portfolio_plugin_pull_base extends portfolio_plugin_base {
 
-    /** @var stdclass single file */
+    /** @var int $file the id of a single file */
     protected $file;
 
     /**
@@ -827,4 +838,36 @@ abstract class portfolio_plugin_pull_base extends portfolio_plugin_base {
         $this->get('exporter')->log_transfer();
     }
 
+    /**
+     * Sets the $file instance var to the id of the supplied \stored_file.
+
+     * This helper allows the $this->get('file') call to return a \stored_file, but means that we only ever record an id reference
+     * in the $file instance var.
+     *
+     * @param \stored_file $file The stored_file instance.
+     * @return void
+     */
+    protected function set_file(\stored_file $file) {
+        $fileid = $file->get_id();
+        if (empty($fileid)) {
+            debugging('stored_file->id should not be empty');
+            $this->file = null;
+        } else {
+            $this->file = $fileid;
+        }
+    }
+
+    /**
+     * Gets the \stored_file object from the file id in the $file instance var.
+     *
+     * @return stored_file|null the \stored_file object if it exists, null otherwise.
+     */
+    protected function get_file() {
+        if (!$this->file) {
+            return null;
+        }
+        // The get_file_by_id call can return false, so normalise to null.
+        $file = get_file_storage()->get_file_by_id($this->file);
+        return ($file) ? $file : null;
+    }
 }
diff --git a/version.php b/version.php
index bcd308b50fc..b631505ddbc 100644
--- a/version.php
+++ b/version.php
@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2017050300.00;              // YYYYMMDD      = weekly release date of this DEV branch.
+$version  = 2017050300.01;              // YYYYMMDD      = weekly release date of this DEV branch.
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.