It is possible in Plupload 3.

.jpg is different from .jpeg, so simply add jpeg to the list of allowed extensions:

filters : {
        max_file_size : '4mb',
        mime_types: [
            {title : "Image files", extensions : "jpg,jpeg,gif,png"} // <--
        ]
},

SC.accessToken() is not available anymore in recent version of SoundClound JS SDK. I've thrown together an example of how it can be done directly without SDK.

http://play.plupload.com/g6aJ2/20

You confuse couple of things here. You cannot use native browser API with those file objects that you receive in your FilesAdded handler. But you can make this work if you use our wrappers, like this:

init: {
 
  FilesAdded: function (up, files) {
      plupload.each(files, function(file, i) {
          var img = new mOxie.Image;
          img.onload = function () {
              // access image size here using this.width and this.height
          };
          img.load(file.getSource());
      });
  }
}

The only thing in Plupload that makes difference between regular files and images is client-side resizing feature. If you have it in the config, remove it and see if it helps. If it - doesn't, then it is something on your server that corrupts the images.

Plupload supports renaming the files, and even auto-generating unique ones, but it doesn't touch extensions. One thing you could probably do, is write your own file re-namer and hook it onto BeforeUpload event.

Not sure what you expect, of course local upload will be much faster than that to the external hosting server. 115mb is pretty big for any connection, so it will take some time to upload. You cannot control it from the config, it is matter of the throughput capability of your internet connection. You could disable chunking, it should make your upload faster... a bit. Also you are using deprecated version of the Plupload. I'd recommend you an upgrade.

Resize happens just before the upload, not on file selection. You can alter options after init using uploader.setOption() method if you are using Core API, or $('#uploader').plupload('option') method for UI Widget.

You can't. Sorry. One file requires at least one request.

You haven't include the code for UI widget (jquery.ui.plupload.min.js). It comes with the package.

You can upload a file on any type and image of any resolution. The constraint exists only for client-side resizing, which is not used by core media uploader in WordPress.

In general post_max_size should be larger than upload_max_filesize - maybe three times post_max_size.

And what exactly doesn't work. Are you getting an error of some kind?