formadd: wrong pointer for file name when CURLFORM_BUFFERPTR used

The internal function that's used to detect known file extensions for
the default Content-Type got the the wrong pointer passed in when
CURLFORM_BUFFER + CURLFORM_BUFFERPTR were used. This had the effect that
strlen() would be used which could lead to an out-of-bounds read (and
thus segfault). In most cases it would only lead to it not finding or
using the correct default content-type.

It also showed that test 554 and test 587 were testing for the
previous/wrong behavior and now they're updated as well.

Bug: http://curl.haxx.se/bug/view.cgi?id=1262
Reported-by: Konstantin Isakov