The .part5 indicates that this is the fifth piece of a larger puzzle.

Many uploaders provide a "checksum" (MD5 or SHA-256) to ensure the file hasn't been tampered with.

When a file is too large to upload to a single server or send via email, users "split" the archive.

To see what’s inside, you cannot just open Part 5. You generally need (Part 1, Part 2, etc.) saved in the same folder to successfully extract the data. Step-by-Step: How to Extract the Content