File extension fix for xml validation example

[hansstevens]

Q	A
Doc fix?	yes
New docs?	no
Applies to	all
Fixed tickets	~

[wouterj]

Good catch! Can you please rebase your PR, it seems to conflict.

If you don't know how to rebase: (assuming origin is this repository and hansstevens is your fork)

$ git checkout improve_doctrine_fileupload
$ git fetch origin
$ git rebase origin/2.3
$ git push hansstevens -f improve_doctrine_fileupload

[hansstevens]

Thanks for your help! It should be ok now..

Regards

[wouterj]

Something was still wrong, so I cherry picked your commit into a new PR: #4995
I've now merged that PR. Thank you for the fix!