From the dropzone documentation (http://www.dropzonejs.com/) there is the accept option -
Dropzone.options.myAwesomeDropzone = {
paramName: "file", // The name that will be used to transfer the file
maxFilesize: 2, // MB
accept: function(file, done) {
if (file.name == "justinbieber.jpg") {
done("Naha, you don't.");
}
else { done(); }
}
};
All you have to do is modify it for your purposes, like accepting files with certain extensions.