On Windows smartphones and PDAs Microsoft's Windows Media DRM is used.

I've also found that on other mobile phones OMA DRM is used. OMA DRM-protected content files have the .dcf extension.
In OMA DRM: FL (Forward Lock) is the DRM method where the content is wrapped into the DRM .DM message and delivered to the mobile device.
The OMA DRM specs could be found on the following web-site:
http://www.openmobilealliance.org/re.../drm_v2_0.html

Here is the thread about .DCF DRM-protected files
drm removal from dcf. extension files


I still can't find anything about .FL files.
FL acronym is used very often. FL could refer to Flash Lite, for example. So you may try to play the file with a flash player.