The documentation for Google App Engine says:
The service accepts image data in the JPEG, PNG, WEBP, GIF (including animated GIF), BMP, TIFF and ICO formats.
It can return transformed images in the JPEG, WEBP and PNG formats. If the input format and the output format are different, the service converts the input data to the output format before performing the transformation.
However, the development server doesn't support ICO out of the box. On startup, it gives this warning:
WARNING: No image reader found for format "ico". An ImageIO plugin must be installed to use this format with the DevAppServer.
So, how do I install a plugin for the DevAppServer? I found https://github.com/haraldk/TwelveMonkeys, which has an ICO plugin, but merely putting it on the webapp classpath is not enough. I can't call ImageIO.scanForPlugins() because ImageIO is not on Google's JRE whitelist.
How do I use ImageIO plugins with the DevAppServer?