If you have a file named "aux" with any extension (eg.: "aux.jpg") and you want to compress it, the system menu says "Add to "_aux.7z" archive" with an underscore before it. Also, the system menu right-clicking on it to the 7z file says "Extract to "_aux\" (with underscore), even if you rename the 7z archive from "_aux.7z" to "aux.7z." When you open the 7z archive, you can see the contained file is named correctly "aux", but when you decompress it the file is renamed to "_aux" (eg.: "_aux.jpg")....