Add ability to customize label contents

Problem/Motivation

We currently use a vertical pipe as the separator between the file extension and size in the contents of label. We need a way to control what separator to use, or to use no separator. We should also support customizing the contents of the label so text can be added if needed.

Proposed resolution

Add a new settings field that supports two "tokens" (not Drupal tokens) to substitute for the file extension and file size. Like [extension] and [size]. The text field should be plain text so it's easy to see the non-breaking spaces that are used by default.

Remaining tasks

• Add a new settings text field for "label contents".
• Adjust process related methods to find [extension] and [size] from "label contents" and replace the tokens with actual data.