Configuring a file uploader

This is pretty easy:

<field id="custom_file_upload"
        translate="label"
        type="file"
        sortOrder="6"
        showInDefault="1"
        showInWebsite="1"
    >
    <label>Logo</label>
    <backend_model>Magento\Config\Model\Config\Backend\File</backend_model>
    <upload_dir config="system" scope_info="1">logo</upload_dir>
</field>

Further reading

• MagePlaza has a great article.