What do the following words have in common?

FILE
MULE
JAY
DALE
MIEN

HINTS:
#1: The same two letters will change each word into a new one.
#2: The two letters are added in the same position in each word.
#3: The two letters are the same.

Answer:

Each word can be changed into a new one by inserting ZZ after the second letter:

FIZZLE
MUZZLE
JAZZY
DAZZLE
MIZZEN