Secondary Replica have two MDF files

Hi, I'm a junior DBA. I have a problem now, I'm using SQL server 2012 enterprise.

I have two windows servers with sql server. I have configured always between these servers,

Now my problem is in primary server every database have one MDF file one and one LDF file. But in secondary server I can see two mdf files.

IE:
In Primary Db
db name : mydb,
db files : mydb.mdf and mydb_log.ldf.

in secondary:
db name : mydb,
db files : mydb.mdf, MYDB_DATA.mdf and mydb_log.ldf.

My question is how this MYDB_DATA.mdf came?