Get normalized document type identifiers
Update retrieval of document type identifiers

It seems that multiple strings can describe a given document type. As a result, we need to flip the array of types in order to keep unique keys.

Additionally, this change adds new types to the list.
Rename `$types` property to `$documentTypes`

At least one other list of ‘types’ will be introduced. The property is thus renamed to be more precise and avoid confusion.