Change error message to localized one

src/user/picture.js

@@ -92,7 +92,7 @@ module.exports = function(User) {
 	User.uploadFromUrl = function(uid, url, callback) {
 		var extension = url.substring(url.lastIndexOf('.') + 1);
 		if (['png', 'jpeg', 'jpg', 'gif'].indexOf(extension) == -1) {
-			return callback('This image type is not allowed');
+			return callback('[[error:invalid-image-extension]]');
 		}
 		var filename = 'uid_' + uid + '_tmp-image.' + extension;
 		downloadFromUrl(url, filename, function(err, downloadedImage) {