The "gcs" plugin makes it work right for storing media.
Not being a wordpress developer (nor an advanced PHP programmer) how does one approach fixing this or asking the authors to fix it? Are they supposed to be using some other wordpress or PHP API for writing and reading files which gets magically transformed into something that just works inside GAE flexible?
So far the two main plugins I have with this issue are Easy Digital Downloads for exporting data, and Mailster for saving message campaigns and storing my own templates. Any advice on how to approach those developers to make their modules more GAE friendly would be appreciated.