.dif = differential backup
.fil = filegroup backup
.trn = tranlog backup
as most people follow MS's lead, as they did with .bak, .mdf, .ndf (silly, imo, why not .mdf also???), etc..
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial:
If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.