Opens a file in the preferred directory for temporary files (as returned by get_tmp_dir) and returns a File and FileIOStream pointing to it.
tmpl should be a string in the GLib file name encoding containing a sequence of six 'X' characters, and containing no
directory components. If it is null, a default template is used.
Unlike the other File constructors, this will return null if a temporary file could not be created.
| tmpl |
Template for the file name, as in open_tmp, or null for a default template |
| iostream |
on return, a FileIOStream for the created file |