Here is how I FIXED it.
First verify that the GD Image Library is disabled.  Got to the component and click on the Information link you will see it DISABLED.

apt-get install php5-gd
/etc/init.d/nginx restart
/etc/init.d/php5-fpm restart

Went ahead and reinstalled the component which was probably not necessary but wont hurt.

Then check again and it is now enabled.

I am trying to upload an image file and I keep getting the error "Not an image file." 

I read the thread above and tried everything to no avail.

My GD Image is enabled, i rebuilt Apache and rebooted the server. Still doesn't work. 


Web server Apache/2.2.23 (Unix) mod_ssl/2.2.23 OpenSSL/0.9.8e-fips-rhel5 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635
PHP version 5.3.21
MySQL version 5.1.66-cll
GD Image library bundled (2.0.34 compatible)
Upload limit 2MB
Memory limit 64M

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I also just encountered this error. I have two websites with FPSS installed, both on the same server. One site would produce this error, the other wouldn't. Both were running the current version of Joomla, 3.6.5. Permissions are correct on both sites. The one that failed with this error message was using FPSS version 3.6.0, the other was using FPSS version 3.7.0.

I debugged the problem and found that the error was due to improper passing of the file parameter to the upload function in /administrator/components/com_fpss/lib/class.upload.php.

I updated the problem site to FPSS 3.7.0 and the problem went away.

JC

Hello John,

If possible update to 3.8 as well.