On Drupal FAPI, '#default_value' is not a valid property for type "file", if you want to use the property #default_value in that case you can use
'#type' => 'managed_file'
and your code should look like this,
$form['file'] = array(
'#title' => t('Upload image'),
'#type' => 'managed_file',
'#description' => t('Images must be one of jpg, bmp, gif or png formats.'),
'#default_value' => $fid, //here you need to provide the file id (get it from database or $file_obj->fid).
'#upload_location' => 'public://'
);