Alfresco allows see preview of not all documents types 
Try any pdf for example - it should work.
Try any pdf for example - it should work.
the problem described below exists for Alfresco 3.2.
decision:
default_preview_flash:
Please correct nodeRef and name in this file.
In web-preview-min set
This decision is the result of looking how things work in Alfresco Share, preview in Alfresco Share and Alfresco plugin for Joomla is basicly the same.
decision:
default_preview_flash:
<?php // no direct acces
defined('_JEXEC') or die('Restricted access');
JHTML::_('behavior.mootools');
$document =& JFactory::getDocument();
//Site-wide YUI Assets
$document->addStylesheet($this->profile->share.'/yui/reset-fonts-grids/reset-fonts-grids.css');
$document->addStylesheet($this->profile->share.'/yui/assets/skins/default/skin.css');
$document->addStylesheet($this->profile->share.'/css/base.css');
$document->addStylesheet($this->profile->share.'/css/yui-layout.css');
$document->addStylesheet($this->profile->share.'/themes/default/presentation.css');
//Common YUI components: RELEASE
$document->addScript($this->profile->share.'/yui/utilities/utilities.js');
$document->addScript($this->profile->share.'/yui/button/button-min.js');
$document->addScript($this->profile->share.'/yui/container/container-min.js');
$document->addScript($this->profile->share.'/yui/menu/menu-min.js');
$document->addScript($this->profile->share.'/yui/json/json-min.js');
$document->addScript($this->profile->share.'/yui/selector/selector-min.js');
$document->addScript($this->profile->share.'/yui/yui-patch.js');
//Site-wide Common Assets
$document->addScript($this->profile->share.'/js/bubbling.v2.1-min.js');
$document->addScript($this->profile->share.'/js/flash/AC_OETags-min.js');
$document->addScript($this->profile->share.'/service/messages.js?locale=ru');
$document->addScript($this->profile->share.'/js/alfresco-min.js');
$document->addScript($this->profile->share.'/js/forms-runtime-min.js');
$document->addScriptDeclaration('
Alfresco.constants.DEBUG = false;
Alfresco.constants.PROXY_URI = "'.$this->profile->share.'/proxy/alfresco/";
Alfresco.constants.THEME = "default";
Alfresco.constants.URL_CONTEXT = "'.$this->profile->share.'/";
Alfresco.constants.URL_PAGECONTEXT = "'.$this->profile->share.'/page/";
Alfresco.constants.URL_SERVICECONTEXT = "'.$this->profile->share.'/service/";
Alfresco.constants.ALF_TICKET = "'.$this->ticket.'";
Alfresco.constants.USERNAME = "";
');
// Document Preview
$document->addStylesheet($this->profile->share.'/components/preview/web-preview.css');
$document->addScript($this->profile->share.'/components/preview/web-preview-min.js');
$document->addStylesheet($this->profile->share.'/components/preview/WebPreview.css');
$document->addScript($this->profile->share.'/js/flash/extMouseWheel-min.js');
//$document->addStylesheet($this->profile->share.'/components/preview/full-preview.css');
//$document->addScript($this->profile->share.'/components/preview/full-preview.js');
?>
<div class="yui-g">
<div class="yui-g first">
<div id="template_x002e_web-preview_x002e_document-details">
<div class="web-preview">
<div class="hd">
<div class="title">
<h4>
<img id="template_x002e_web-preview_x002e_document-details-title-img" src="/share/components/images/generic-file-32.png" alt="File" />
<span id="template_x002e_web-preview_x002e_document-details-title-span"></span>
</h4>
</div>
</div>
<div class="bd">
<script type="text/javascript">//<![CDATA[
new Alfresco.WebPreview("template_x002e_web-preview_x002e_document-details").setOptions(
{
nodeRef: "workspace://SpacesStore/e0a08e88-e2e0-4adf-af6c-b0dd55efc045",
name: "content.Alfresco Share.pdf",
icon: "/components/images/generic-file-32.png",
mimeType: "application/pdf",
previews: ["imgpreview", "avatar", "doclib"]
}).setMessages(
{"preview.actualSize": "Actual Size", "label.noFlash": "To view the preview please download the latest Flash Player from the<br\/><a href=\"http:\/\/www.adobe.com\/go\/getflashplayer\">Adobe Flash Player Download Center<\/a>.", "preview.fullscreen": "Fullscreen", "preview.fitHeight": "Fit Height", "error.io": "The preview could not be loaded from the server. ", "preview.fitPage": "Fit Page", "preview.pageOf": "of", "error.content": "The content cannot be displayed because it is not of type png, jpg, gif or swf.", "label.noPreview": "This document can't be previewed.<br\/><a class=\"theme-color-1\" href=\"{0}\">Click here to download it.<\/a>", "preview.page": "Page", "preview.fitWidth": "Fit Width", "preview.fullwindowEscape": "Press Esc to exit full window mode", "preview.fullwindow": "Maximize", "error.error": "The content cannot be displayed due to an unknown error."}
);
//]]></script>
<div id="template_x002e_web-preview_x002e_document-details-shadow-swf-div" class="preview-swf">
<div id="template_x002e_web-preview_x002e_document-details-swfPlayerMessage-div"></div>
</div>
</div>
</div></div>
</div></div>
<div id="alfresco-yuiloader"></div>
<script type="text/javascript">//<![CDATA[
Alfresco.util.YUILoaderHelper.loadComponents();
//]]></script>
Please correct nodeRef and name in this file.
In web-preview-min set
q.addParam("allowScriptAccess","always").This decision is the result of looking how things work in Alfresco Share, preview in Alfresco Share and Alfresco plugin for Joomla is basicly the same.
the demo site work perfectly whith alfresco 3.1 … i don't understand why they don't publish his code or upgrade the plugin. 
