Hi Leandro,

This can be done by editing the lightbox javascript, which is located in the ~\CMSScripts\lightbox directory, so please open the lightbox.js file, go through its source code until you find the updateDetails: function(). This is the place, where this javascript creates a caption for the image, so here you can remove that extension:

// if caption is not null
if (imageArray[activeImage][1]) {
Element.show('caption');
Element.setInnerHTML('caption', imageArray[activeImage][1].substring(0, imageArray[activeImage][1].lastIndexOf('.')));
}
else {
Element.show('caption');
Element.setInnerHTML('caption', '&nbsp');
}

Best regards,
Jan Hermann