$attachments = $resource->getTVValue('eletterAttachment');
// returns: about/image.jpg
$tv = $modx->getObject('modTemplateVar', array('name'=>'eletterAttachment'));
$attachments = $tv->renderOutput($resource->get('id'));
// also returns: about/image.jpg
This question has been answered by jgulledge19. See the first response.
$tv = $modx->getObject('modTemplateVar', array('name'=>'eletterAttachment'));
$tvValue = $tv->getValue($resource->get('id'));
$source = $tv->getSource();
$source->initialize();
$path = $source->getBasePath();
$attachments = array(str_replace(MODX_BASE_PATH, '', $path).'/'.ltrim($tvValue,'/'));