Message: Attachments are being removed and/or cleaned...

If it removed them there may have been a reason, it could be a virus, or it could be a suspicious script, or.... However, you could see if it has placed them in its quarantine or backup storage. Open Nav, click reports, click on the view report next to quarantined items. In the left pane, click on qarantined items, see if any are there in the right pane; repeat for Backup items. If they are there you can restore them (highlight one, action menu, restore).

BUT before you do that, you should also look, on the reports page, click view report next to activity log. Click on Virus alerts; If you highlight any item you can see where it came from and why NAV excised it (most of mine are viruses from emails, but a couple are labelled suspicious script, and highlighting tells me what that they came from particular downloads.)

You can prevent NAV from automatically quarantining items it finds during a manual scan: Options, manual scan, check the radio button ask me what to do. Then when it comes to one of your htm attachments, you'll have the chance to tell it to ignore it.

If they're all related to script blocking, you can turn that off or tell it to ask you what to do, then that will also prevent them being ripped out in background scans.

(I hope this makes sense, it's a bit late at night where I am).