Verify code samples for built in umbraco property editors

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The following V15 articles have code samples, that might be out of date or plain out of order:

• 15/umbraco-cms/fundamentals/backoffice/property-editors/built-in-umbraco-property-editors/file-upload.md

• 15/umbraco-cms/fundamentals/backoffice/property-editors/built-in-umbraco-property-editors/multi-url-picker.md

• 15/umbraco-cms/fundamentals/backoffice/property-editors/built-in-umbraco-property-editors/rich-text-editor-tinymce/README.md

Remember to also update the V16 counterparts:

• 16/umbraco-cms/fundamentals/backoffice/property-editors/built-in-umbraco-property-editors/file-upload.md

• 16/umbraco-cms/fundamentals/backoffice/property-editors/built-in-umbraco-property-editors/multi-url-picker.md

• 16/umbraco-cms/fundamentals/backoffice/property-editors/built-in-umbraco-property-editors/rich-text-editor-tinymce/README.md