This is used to upload a file to the repository as a node of type
nodes:file.
You may specify:
- The name of the file in the repository
- Of course, you must select the file from the local filesystem.
- Optionally, you may add a content type (= mime type). This may be
helpful, if the browser has special support for displaying files of
that type. Some types will be determined from their file extension
by the application, but you can change the setting.