Unsupported output format "gif"

[tuananh]

I have an image which is jpg but somehow when i use external source to resize it, I got Unsupported output format "gif"

The image url is

http://pix3.agoda.net/hotelimages/694/69405/69405_14040308360018949836.jpg?s=312x

env EXTERNAL_SOURCE_TEST='http://pix3.agoda.net and access it via

example.com/etest/hotelimages/694/69405/69405_14040308360018949836.jpg?s=312x

[teohhanhui]

Somehow it's being identified as a 1x1 GIF.