Some improvements

https://github.com/odlex/bolt/commit/9ea73a0ceed3f3430ae80d67c458f19d998220c5 : add support for taxonomy default value when options is set in yml

https://github.com/odlex/bolt/commit/99920b08d2459270c799c60c00367d42cb0e2e82 : Attempt to establish a test on the fileupload (js) to allow only certain
file type. Using the keyword "extensions" word in the declaration of the fields in
contenttypes.yml

https://github.com/odlex/bolt/commit/efd3583973a5cace6c492333fb87c28180dc07fe: corrections semantic errors