Dialog description:
There are two lists with encodings (all encodings, supported by Qt): input encoding and output encoding. The process of encoding detection is manual. You can go through the list till the text, displayed in preview, becomes sensible. Use appropriate spinbox to increase number of lines to preview if needed.
Then you can select output encoding and save input file as a new one (or overwrite input file) using selected encoding.
If this dialog is opened before text conversion, button "Read" appears instead of button "Close". Press it to proceed after you have detected right encoding.