There is only one file type that uses the .LASTLOGIN extension:
Minecraft User Credential File
Encrypted file containing the username and the password credentials of the last user to use the launcher to log into the Minecraft game; found in C/Users/User/AppData/Roaming/minecraft.
The LASTLOGIN file is usually encrypted using DES in CBC mode with PKCS #5 padding, however, if a user's Java platform does not support the described method of encryption, their username and password will be stored without any encryption whatsoever.
The password in the LASTLOGIN file is left blank if the user opted not to have the launcher remember their password.
Updated: July 14, 2013