Bit of jQuery code for keeping suitable filename extension in MediaWiki file upload form. Useful when used with default filename parameter in Semantic Forms http://www.mediawiki.org/wiki/Extension:Semantic_Forms/Defining_forms

keep_filextension_mwupload.js

/** Make filename readonly, so filename is either the default one defined in the form or the filename in your drive **/
$("#mw-upload-form #wpDestFile").attr('readonly', true);
 
/** Keep file extension in upload. Default filename is changed according the filename extension uploaded **/
$("input:file").change(function (){
    var filename = $('input[type=file]').val().replace(/C:\\fakepath\\/i, '');
    var extension = filename.substr( (filename.lastIndexOf('.') +1) );
    var subs = "."+extension;
    var filenamef = $("#wpDestFile").val();
    var extensionf = filenamef.substr( (filenamef.lastIndexOf('.') +1) );
    var init = "."+extensionf;
    var filenamev = filenamef.replace(init, subs);
    $("#wpDestFile").val(filenamev);
});