OPEN DATASET P_FILE FOR INPUT IN BINARY MODE

hiiii

i have a question for this piece of code:

OPEN DATASET P_FILE FOR INPUT IN BINARY MODE

the value in P_FILE = /usr/sap/tmp/file

i get sy-subrc 8

when i put value

the value in P_FILE = /usr/sap/tmp/file.dat the sy-subrc = 0.

is this an error in code or when we do the text we must put the .dat extension

because the unit test sript said to run the test without the .dat extension