ISLANDORA-2153: Allow datastreams with no MIME type restrictions to be replaced with any file type (#1)

* Return empty if no restriction set, enforce this on the form

includes/datastream.version.inc

@@ -283,7 +283,13 @@ function islandora_datastream_version_replace_form($form, &$form_state, Abstract
   $form_state['object'] = $object;
 
   $extensions = islandora_get_extensions_for_datastream($object, $datastream->id);
+  if (empty($extensions)) {
+    // In case no extensions are returned, don't limit.
+    $valid_extensions = NULL;
+  }
+  else {
     $valid_extensions = implode(' ', $extensions);
+  }
   $upload_size = min((int) ini_get('post_max_size'), (int) ini_get('upload_max_filesize'));
   return array(
     'dsid_fieldset' => array(

includes/mimetype.utils.inc

@@ -76,6 +76,10 @@ function islandora_get_extensions_for_datastream(AbstractObject $object, $dsid)
   module_load_include('inc', 'islandora', 'includes/utilities');
   $datastream_mime_map = islandora_get_datastreams_requirements_from_models($object->models);
   $mimes = isset($datastream_mime_map[$dsid]) ? $datastream_mime_map[$dsid]['mime'] : array();
+  // If no restriction set via DS-COMPOSITE-MODEL return an empty array.
+  if (empty($mimes)) {
+    return array();
+  }
   if (isset($object[$dsid])) {
     $current_mime = $object[$dsid]->mimetype;
     if (!in_array($current_mime, $mimes)) {