I figured out this problem. I know this is an old post, but I thought you might want to update the plugin so it won't be a problem again. In the plugin you do a chg_mod on the file to 0444 which is read only but unlink doesn't seem to want to delete a read only file. Therefore, if you chg_mod to 0777, you won't have a problem.
In my hack on your plugin, I set the original file_mode to 0777 for newly written files and then also did the chg_mod 0777 on each file before delete in the remove and cleanup functions.
I hope you fix this bug in your plugin.