allowed file extensions for cms resources

[NKjoep]

A great feature to implement would be the possibility to configure the allowed extension for Attachment files.
Actually as we know the allowed extensions are defined in

entando-core-engine / src / main / resources / spring / plugins / jacms / aps / managers / resourceTypesDef.xml

(https://github.com/entando/entando-core-engine/blob/master/src/main/resources/spring/plugins/jacms/aps/managers/resourceTypesDef.xml#L31)

<bean id="jacmsAttachResource" class="com.agiletec.plugins.jacms.aps.system.services.resource.model.AttachResource" parent="jacmsAbstractResource" >
    <property name="type" value="Attach" />
    <property name="folder" value="cms/documents" />
    <property 
        name="allowedExtensions" 
        value="pdf,xls,doc,ppt,txt,rtf,sxw,sxc,odt,ods,odp,tar,gz,zip,rar,flv,swf,avi,wmv,ogg,mp3,wav,ogm,mov,iso,nrg," />
</bean>