Simple PG Sql question (i hope)

I have a simple query:

select true,78,'Here is a value' as stringfield, testname from tbltest;

it returns:
true as bool
 78 as int4
and the string 'Here is a value' as Unknown

Why is it that all the values besides the string come back with the
correct type?
what is a simple string being returned as unknown instead of varchar or
text?


Thanks,

Tony