What are the mime-types associated with the files? Thunar can be configured to display MIME Type via View->"Configure Columns". On my system, a sample .doc file is shown as application/msword while a sample .txt file is shown as text/plain.
The system identifies files as particular MIME types according to the definitions in files in places that very from distro to distro and desktop to desktop.
If this problem is affecting only one user, you could try (as the user) to find the user-specific mime configuration files by
find ~ \( -type f -o -type d \) -iname '*mime*'
If you find any you could try renaming them make them inaccessible and restarting your desktop session. If the problem disappears it was caused by one of the files in there.
If the problem does not disappear then it is probably caused by system-wide mime files which would require more careful investigation. You could try (as root) to find them by
find / \( -type f -o -type d \) -iname '*mime*'
The system identifies MIME types by several characteristics (I know only a little about this) including extensions, as defined by files in the mime/glob* directories, and by looking into the files looking for "magic numbers", characteristic strings ...