Allow renamed files to be lowercase and without spaces

Currently when you rename an uploaded file using a token from a textfield, the renamed file can have uppercase letters, spaces, etc. and there's no way to change this.

I notice that Webform uses transliteration_get() for the renaming if the Transliteration module is enabled, however, since Transliteration has a setting that allows uploaded files to be renamed to lowercase, I think Webform should adhere to this setting and therefore use transliteration_clean_filename() for renaming instead.