.REM files are Blackberry files that have been encrypted using the Blackberry device's encryption option. The REM extension is appended at the end ex <filename>.jpg.rem
Depending on how to device was encrypted you may or may not be able to open a Blackberry REM encrypted file. When Blackberry encrypts files it uses a randomly generated Device Key which is stored in its RAM. If the device is wiped or re-installed, RAM is cleared and the key will no longer be available.
If files were encrypted using a User or Device Password, this will be required to open a Blackberry encrypted REM file.