Unable to write to /tmp when uploading WAV file for recording in UCP

I’ve got an issue when trying to upload recordings via UCP (Chrome). When I upload the file, I think the portal tries to convert it to .sln and is unable to save or read the temp file. I get errors like this in the console:

[2015-10-21 14:26:17] WARNING[8087] file.c: Unable to open /tmp/temp.1445451976644.sln48

I searched around and found someone with the same issue in the IRC logs but no answer there. I’ve checked the permissions on my /tmp directory which are “default” for Centos7 (1777) or “drwxrwxrwt.” as per http://unix.stackexchange.com/questions/71622/what-are-common-rights-for-tmp-i-unintentionnally-set-it-all-public-recursive

I have also tried fwconsole chown followed by a reboot but that did not work either. Any ideas?

The file is uploaded to /tmp but Asterisk is unable to convert it. This was previously explained in IRC but was not listened to. I know this because it wouldn’t hit the code in file.c if the file never existed (there is a file exists check first and fore-most)

Thanks Andrew - what do you recommend as a fix? Is this a permissions issue or is there a missing binary?