Returns the file type to use for saving a document.
- iOS 10.0+
A Uniform Type Identifier (UTI) identifying a document type (for example, PDF or HTML).
The default implementation returns the current file type obtained from the
file property. The default implementation of the
save(to: method appends an extension to the file URL that is based on the file type. So if you want to move the document to a new type and extension, you can override this method to supply that file type.