Likewise both drag&drop and click to bring up the file browser give the “The image could not be uploaded. Please check that it is a valid JPEG, PNG, or GIF file and that the file size is no larger than 128 MB.” message. From a Mac. Using old Firefox 45.9.0 (that’s what “ESR” really means!-). Via a squid proxy. [Edit: FF 48.0.2, no proxy; still NG. .jpg, .png, or .gif, all same. Safari 13 Mac, no joy either. Likewise a .jpg downloaded from the forum then attempted to re-upload, same NG.] (If any of that matters, couldn’t find the “other thread” mentioned, to hunt for work-arounds.) [Edit: As suggested, attaching a file to a message to be sent appears to work, but of course it isn’t trying to parse any image format.]
