Preview for text fields

Could you implement a preview for text fields, browseable in the same way as the picture preview? It could be only read-only.

Support for formats of data commonly stored in text fields would be ideal. HTML preview would be definitely the most usable one, but there could be also some preview for XML data, JSON data, serialized PHP objects etc.

Alternativelly, add at least the option to preview a text field in user defined external program (text editor, internet browser...).