Just completed an upgrade to 4.5.9 Network edition from 4.5.7 OSS - very smooth in general.
The "view as html" only tries to download the attachment for most types. .PDF, .doc, .xls all are only downloaded.
But, .txt will render as html. Tested with PDF's from before the upgrade and ones sent after - still just downloads.
Ran the Network trial back under 4.5.6 and this feature worked fine.
Logs for convertd look clean