Can't open attached files directly

When I try to open an attached file in an email message (say "Example.pdf") I get this error message, or something similar:

"An error occurred while opening a document from the file:
C:\Users\CHS2\AppData\Local\Temp\nsnD5B3.tmp\ContainedTemp\pid-20128\Example.pdf"

Based on ChatGPT, I tried adding a line like:

user_pref("browser.cache.disk.parent_directory", "D:\\cDat\\Programs\\TBird\\Temp");

but Thunderbird Portable overwrites the line if it's in prefs.js and ignores it if it's in user.js

Is this an inherent problem with Thunderbird Portable that can't be fixed?