Add support for FileField to custom_serializer

[raulcodes]

Breaking change

Proposed change

In order to build config flows that support uploading files, the custom_serializer should be able to serialize FileField.

This PR depends on this to build a config flow that supports iCalendar files.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue: Corresponding frontend PR
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
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[bramkragten]

We should use the file selector for this.