Add iconClass config option for fileupload formwidget

[mjauvin]

This PR adds the iconClass config to the fileupload formwidget when mode = file. This complements the existing prompt config.

The config below:

fields:
  upload:
    type: fileupload
    mode: file
    prompt: Attach File
    iconClass: icon-paperclip

changes the formwidget look from this:

To this:

[Copilot]

Pull Request Overview

This PR adds a new iconClass configuration option to the fileupload formwidget when mode is set to file, allowing developers to customize the icon displayed on the upload button. The change provides more flexibility in the visual appearance of file upload widgets.

• Adds iconClass property to the FileUpload class with proper documentation
• Updates the template to use the configurable icon class instead of hardcoded 'icon-upload'
• Implements a fallback to 'icon-upload' when no custom icon class is provided

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
modules/backend/formwidgets/FileUpload.php	Adds iconClass property, configuration loading, and variable preparation with fallback
modules/backend/formwidgets/fileupload/partials/_file_single.php	Updates template to use configurable icon class variable

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[LukeTowers]

LGTM, can you submit a docs PR for this as well?

[mjauvin]

LGTM, can you submit a docs PR for this as well?

wintercms/docs#247

[LukeTowers]

@mjauvin question I just thought of, does changing this change the icon injected into the prompt if the prompt is left as its default value?

[mjauvin]

No, it doesn't.

[mjauvin]

I'll submit a PR to add this