[TxMt] Using „First Line Match” in grammars

[Stefan Daschek]

When and how is the “First Line Match” setting of language grammars 
considered?

In my experiments it doesn't seem to have any effect at all: For 
example, I'd like TextMate to use the RSpec grammar for all *.rb having 
a first line matching '.*spec_helper.*', but it seems to always use the 
Rails grammar instead (even for newly created files). I suspect TextMate 
remembers some kind of manual override – if so, how can I reset this 
override? Is there any way to see (debug) how TextMate choses the grammar?