I was given two files without extensions and was told to decrypt them.
Both ZIP and JPG files have well known magic numbers at the start of the file. For examples, ZIP files start with the two letters "PK" (most likely as the initials of Phil Katz, the developer of PKZIP), followed by the bytes 03 04 to indicate a "file header".
If all you know is that the files are an encrypted ZIP file and a JPG image, probably a very simple cipher like bytewise XOR or addition of a constant to each byte is used to keep the puzzle solvable. The well-known bytes at the file start allow you to deduce the key. This kind of attack is called a "known-plaintext attack".