Form images validator.
Searches for file inputs in a form and validates their file extensions. For a passed/failed validation, a provided callback will be invoked.
The validation is triggered on form submit.
Consider the respective library file for custom callback method definitions.
Possible optional options are:
- valid_extensions {String} Valid file extensions (pipe-separated, e.g.: 'gif|jpg|tiff')
- selector {String} File input selector.