Rails upload magic number?

From Rails wiki: http://wiki.rubyonrails.com/rails/pages/HowtoUploadFiles
“The uploaded file will be a
-like 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 binary 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?