Caja changes filetype to text/plain on thumbnail generation and tries to open it in Pluma

[Norbert]

Steps to reproduce:
1. Open Caja
2. Navigate to some folder (for example, ~/Documents)
3. Open LibreOffice Writer
4. Save document in LibreOffice Writer as for example ~/Documents/test.docx (see caja-docx_after_first_save.png screenshot of Caja)
5. Try open ~/Documents/test.docx by double click on it from Caja and get error message

"Could not open the file /home/ltsp157/Documents/test.docx.

pluma has not been able to detect the character encoding.
Please check that you are not trying to open a binary file.
Select a character encoding from the menu and try again.
"

6. Click Reload in Caja window, open ~/Documents/test.docx, it will open normally in LibreOffice Writer.

Expected results:
* Caja thumbnail generation works normally, step 5 does not appear

Actual results:
* Caja thumbnail generation confuses user by showing him/her "corrupted" preview and trying to open the file in Pluma as plain text.

Note:
* this problem exists on both local and remote filesystems (such as SSHFS)
* DOC, OpenDocument (.odt, .ods) files are affected too

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: caja 1.12.7-1
ProcVersionSignature: Ubuntu 4.4.0-97.120-generic 4.4.87
Uname: Linux 4.4.0-97-generic i686
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: i386
CurrentDesktop: MATE
Date: Thu Dec 28 10:32:51 2017
JournalErrors:
 Error: command ['journalctl', '-b', '--priority=warning', '--lines=1000'] failed with exit code 1: Hint: You are currently not seeing messages from other users and the system.
       Users in the 'systemd-journal' group can see all messages. Pass -q to
       turn off this notice.
 No journal files were opened due to insufficient permissions.
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: caja
UpgradeStatus: No upgrade log present (probably fresh install)

[Norbert (nrbrtx)]

Can you reproduce this bug with newer version of Ubuntu MATE?

[Launchpad Janitor (janitor)]

[Expired for caja (Ubuntu) because there has been no activity for 60 days.]