Rails upload magic number?

From Rails wiki:
“The uploaded file will be a
object (if over 10kb in size). These can be copied by the filesystem
instead of being read and processed through Ruby.”

Basically, Rails makes a temporary file instead of having all that
data in your memory when uploading big files. I have no problem handling
either situation, but it would be nice to know the exact number at which
this change in treatment happens (it’s not exactly 10kilobytes or
10kilobits.) It seems like this should be a constant, if anything…does
anybody know either a number for this, or better yet, a constant?

Larry Diehl