SEQ: do not automatically pick up files with the .seq suffix

Reported by Stéphane Dallongeville.
SEQ: one more type checking fix

See 063afd4cc7003efd0e596979035d13f44f56dbeb.  This makes
isThisType(RandomAccessInputStream) a little more strict, to make up for
the fact that the suffix is no longer a sufficient indicator of type.
Merge pull request #581 from melissalinkert/rebased/dev_4_4/seq-suffix

SEQ: do not automatically pick up files with the .seq suffix (rebased onto dev_4_4)