This is mostly for editor fields, to fix the problem that components were
edited by the text editor and not the code editor.

Editor is selected by Base_getEditorFieldPreferredTextEditor, which will
use a text editor by default, unless the document has a content type, in
which case it will return the most appropriate editor, which is by default
the source code editor for documents whose text content is not text/html.

It's anyway not wrong to have a content type defined here, Components
are TextDocuments and TextDocuments usually have getContentType method.