I've created an 'alpha' version of the grid format for Moodle 2.4 available from:
I've made it work and accept png files but for some unknown reason the images do not display. In working though the code (editimage.php), they are uploaded to the user's draft area and converted successfully with an entry in the 'files' table and physically stored in the 'filedir' folder within the moodle data folder. And changing the extension to the filename that matches the filename hash to the file type, i.e. '.png' gives the ability to preview the file and confirm it is ok. Hacking '/lib/filelib.php' - 'readstring_accel' function and changing the mime type to text makes the browser display the returned bytes as text and this shows that indeed an image is being transferred. Accessing the link directly (i.e. 'http://localhost/moodlegjb/pluginfile.php/15/course/section/2/Columns_Screen_Shot_24_edit.png') also fails with FireFox reporting that the image is corrupt.
I'm stumped and not sure if it is my set up or not. I have cleared browser caches, operated a 2.3 version that works, compared changes in '/lib/filelib.php' and run a virtual linux machine with chrome and firefox browsers.
Any help appreciated.