A .rar file is a proprietary archive format that supports data compression, error recovery, and file spanning . It is widely used to bundle multiple large files into a single, smaller package for easier distribution.
The file likely contains a spreadsheet (Excel/CSV) or a database file listing postal codes. The "New folder" designation suggests it was hastily packed or organized by an automated system or a user sorting data.
: Use the national postal service website for the specific country you are researching.