[1.2.X] Fixed #11159 -- Added mimetype detection to the test client for file...

[1.2.X] Fixed #11159 -- Added mimetype detection to the test client for file uploads. Thanks to notanumber for the report and patch, and lomin for the test case.

Backport of r13517 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13518 bcc190cf-cafb-0310-a4f2-bffc1f526a37