WYSIWYG image upload fail on Jetty

To reproduce:

• start a jetty+hsqldb all in one demo package
• edit or create a page with the WYSIWYG
• click on the insert picture icon in the menu bar
• choose the tab upload
• click on browse
• select an image (for example, the logo-x-200.png image attachment to this issue)
• press the upload button
• click on select
• you see the edit image dialog, just click on insert
• click save

The content does contain an "image", but the uploaded file is broken (see fv1.1.png for more details). It seems to have the expected size, but it's unreadable (it seems the second half of the file is filled with 0).

It works just fine with the link tool (choose attachment, and upload), so it's really specific to the WYSIWYG image tool, apparently.

The behavior is also slightly different depending on the browser: when uploading with Firefox the image file is totally broken (impossible to read it), with Chrome we can see half of the expected image. But it seems to be more a difference in how Firefox and Chrome can read broken image files.