More robust image file validation
Previously, it was possible to upload PSD files by simply renaming them to have a '.jpg' extension.

This causes issues as wagtailimages cannot resize PSD files causing crashes.

This commit fixes this by validating that the internal format of the file matches the extension.
Merge pull request #252 from kaedroho/image-validation-fix

More robust image file validation
Reopen images for validation if they are closed
Merge branch 'image-validation-fix'
Revert "Reopen images for validation if they are closed"

This reverts commit 7d43b1cf6eda74c86209a4cae0d71557ce9bdbc0.
Don't check internal file formatl if image is closed
Merge branch 'master' of github.com:torchbox/wagtail into docs
editing api started