You can see that the directory upload has the following permissions : 775 (you can be less permissive, of course   ), and the owner is submarine (the account I use to upload icons via ftp), and the group is www-data (Apache user on Debian).

Permissions on files - bis

And in the “uploads” directory, you can see that all the icons have been uploaded, so the permissions are : 644, the owner & the group are submarine.

Permissions on files - ter

The plugin has the following permissions, too (I have uploaded it via ftp): 644, the owner & the group are submarine.

If you have problems with the permissions, try first 777 on the directory and the files.