Disable image file name correction by default. Closes #3332

To enable image file correction, the option `correct_image_extensions`
can be set to `true`.
This applies the exif_imagetype function on all files if available and
corrects the file extension if it doesn’t match.
Use https for the demos.
Revert 5ab59fa..243084e

This rolls back to commit 5ab59faeafbc06fe60604ea9542174ad3662ffb6.
Merge branch 'master' into gh-pages