This file contains a long series of hexadecimal characters.

This is a hex dump. We can reverse it xxd like this:

xxd -p -r mystery.file > binary.data

Once you have the binary file, you can use the file command to check its file signature.

file binary.data

It is a .png image. Rename it.

mv binary.data picture.png

Open it up using the file browser: (Flag Redacted)