The strange case of uneditable files and odd formats
Symptom
Some uploaded files (i.e. the Plone File content type) aren't editable after creation. The edit tab is visible, but clicking on it takes you to the That content does not exist error page. The file objects are well behaved in all other cases. Uploading new files showed that this behaviour was ongoing but didn't happen for every uploaded file.
Diagnosis
There was no clear pattern in the name, title, id or date of files. Going into the ZMI showed a puzzling pattern. Every file for which editing worked was displayed with the normal file (page) icon. Every file for which it didn't work had a custom icon, the ATFile or PDF one. Examining the file metadata itself showed the important difference. Working files had the format application/octet-stream. Problematic files had something else: application/msword or application/pdf.
Solution
For each of the problem files, change their format to application/octet-stream.
Cause
Unknown. This Plone system has been through the wars (some crude hacking, malformed products and the occasional corrupted object) but nothing leaps out as obviously broken. As a guess, maybe portal_transforms is implicated somehow.
Technical details
OS: SuSE linux SLES 9; Python: 2.3.5; Zope: 2.7.8; Plone: 2.1.4.