[1.2.X] Fixed #12955 -- Use the size attribute of the file instead of len() to...

[1.2.X] Fixed #12955 -- Use the size attribute of the file instead of len() to support uploads larger than 2GB. Thanks, swiedenroth and isagalaev.

Backport from trunk (r14878).

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14903 bcc190cf-cafb-0310-a4f2-bffc1f526a37