[Refactor] Align extension checks

[fenekku]

To determine "previewability" 2 approaches are taken relying on 2 different formats for file extensions

1. .txt, .zip, .png ...
2. txt, zip, png, ...

Before showing previews

has_previewable_files calls

    is_previewable(ext) calls

        current_previewer.previewable_extensions

            loads previewable_extensions from each plugin 

This checks extensions in this format txt, md, zip...

When actually previewing

preview calls

    plugin.can_preview calls

        file is a PreviewFile instance

        file.has_extensions(
            *[f".{ext}" for ext in previewable_extensions]
        )

checks extensions in this format .txt, .md, .zip...

This minute difference can easily breakdown the preview and requires more maintenance.