Description of problem:

SElinux does not allow Thunar to create thumbnail images in ~/.cache/thumbnails

Version-Release number of selected component (if applicable):
x86_64 Fedora 23 Xfce, 4.3.5-300.fc23.x86_64

How reproducible:

Open a folder with images in Thunar: thumbnails are not displayed. 
"setenforce 0" fixes the issue. 

Just in case: I was opening a folder on a manually mounted non-system NTFS hard drive. 

Comments:
In order to fix the issue, several different permissions must be set. Check the attached thumbs-pol.te for details. I do not know how to set them all at once, other then use the attached thumbs-pol.pp. In my case, I was repeating the steps below. 

Fix:
Modify selinux policy via repetitions of the following steps:

0. Run "setenforce 1" to go back to enforcing mode.

1. Open a folder with images in Thunar (it is not required to restart thunar, just go to parent dir and back)

2. (optional): check /var/log/audit/audit.log; there must be a line like:
type=AVC msg=audit(1456332407.222:284): avc:  denied  { add_name } for  pid=1369 comm="pool" name="0dc587ea8551e5fcdb39b3599e9d4d68-1456332407-223458.png" scontext=unconfined_u:unconfined_r:thumb_t:s0-s0:c0.c1023 tcontext=unconfined_u:object_r:unlabeled_t:s0 tclass=dir permissive=0

3. grep <keyword> /var/log/audit/audit.log |audit2allow -M thumbs-pol

As there are several permission to set, the lines in audit.log will differ after policy update. However, those must contain a common keyword. In my case, it was 'comm="pool"'. My command looked as:
grep 'comm="pool"' /var/log/audit/audit.log |audit2allow -M thumbs-pol

4. Update policy via:
semodule -i thumbs-pol.pp 

5. Go to step 1 and check if the thumbnails are displayed, continue to step 2 or 3 if not. Note that the type=AVC messages will differ.