Typically:
.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) A socialist is someone who will give you the shirt off *someone else's* back.