I had originally posted this under the Attachment error from MAPI, because I thought it was similar (based on the listing of Temporary Internet Files/OLK), but now I realize it must be a different issue.
My company uses Seagate Info, to generate reports. Some of our users do not have the full client components for Info installed, they just use the stand alone Info Analyzer as a viewer.

These users cannot open an RPT attachment (Native Crystal Reports or Seagate Info file) from within Microsoft Outlook 2000 since we upgraded to Info 7.5.

We were saving the RPT files to an Exchange Public Folder, but we have also tried emailing the reports to these users. If they double-click the file and choose open, it fails with:

"Can't open this item. The file, 'C:WINNTProfiles{username}Temporary Internet FIlesOLK60{filename.RPT} does not have a program associated with it for this action. To create an association, double-click My Computer on your desktop. On the View menu, click Options, then click File Types.'

I have compared the file association for view and open on some of these machines with machines that work, and I cannot see any differences.

Since they can choose save, and open the file from their local hard drive by double-clicking it, it seems that the file association should be correct.

Previously they were able to open the files in the Info Analyzer from within Outlook before we upgraded to Info 7.5.