Creates a web-accessible URL for a stream to an external or local file.
Compatibility: normal paths and stream wrappers.
There are two kinds of local files:
- "managed files", i.e. those stored by a Drupal-compatible stream wrapper.
These are files that have either been uploaded by users or were generated
automatically (for example through CSS aggregation).
- "shipped files", i.e. those outside of the files directory, which ship as
part of Drupal core or contributed modules or themes.