I've set up a security group with access to files and images (asset admin), access to site content and access to all CMS interfaces but when logged in this security group is not able to upload images via the Files & Images Upload tab, I just get an error "File is not a valid upload".
This group can upload if I change it's permissions to full administrative rights, which obviously I don't want to do.
I've seen other posts on the forum relevant to earlier versions of ss, I'm using 2.3.1 and the fixes don't apply/work.

I am having the same issue and not finding anything to work. Has anyone found a fix for this?

i'm having the same problem (SS 2.3.3).
(How) did you solve it?

We never have... just ended up giving the client full access to the admin. I don't know what we would do in a multiple user scenario. This is a big bug in my opinion and am disappointed that the newest version didn't correct this.

Hi,

Did you guys ever find your answer to this? If not then this may help http://ssorg.bigbird.silverstripe.com/all-other-modules/show/271768#post271768

I have the same problem (v. 2.3.1).

The developer working with me found that commenting the Validate function in the sapphire/filesystem/upload.php file did the trick, but we can't remove this security function !!!
(Info : this function checks if the file was sent via http)

The question is : why does the permission interfere with this function ?

Having the same problem.

Unless the user doesn't have Full Admin right, when I try to upload something, I get the 'file is not a valid upload' error. Afterwards, the screen is messed up (see screenshot) it seems that all tabs are displayed at once. The strange thing is that it remains that way, even if I log in with another user that has Full Admin Rights. Closing and reopening the browser fixes the display problem.

I don't have this problem with the site on my local server.
The online server does have safe_mode enabled.

The exact same problem bugs me, I have had to resort to giving all content editors full admin-rights... hope they don't break anything. :-(