That could be just a typo but, there are two realistic possibilities here, "sequential" and "sequence".
Either is correct depending on the exact circumstance.
If the files have on order themselves, just as numbered jackets or dates, then:
The files are in sequential order.
Meaning they are in the order printed on them, 1, 2, 3, 4, etc...
However, if the binders track something else that is called a 'Sequence' on it's own, then "sequence order" is correct. For example:
We track every Production Sequence on paper. The binders are on that shelf in sequence order.
Meaning they are order by Production Sequence, not their own order.