Question

plupload creates nice ids in file object. How this id can be sent to the upload script?
The upload script has 3 variables in $_POST – file name, chunk number and total number of chunks.

How to add another parameter to plupload’s POST request (in my case, the file.id)?

Answer 1

The first step would be to add a handler to the BeforeUpload event.

Then, if you are using multipart, you can change the uploader settings to dynamically set different multipart params:

plupload_instance.bind('BeforeUpload', function (up, file) {
    up.settings.multipart_params = {fileid: file.id}
});

(warning: this example overrides any and all multipart_params, you can play it smarter than that by just setting fileid)

if you are not using multipart, your only options would be to pass the argument as a header, or to manually add the param to the URL for each file (these 2 options should also be done within BeforeUpload).
Note that when not using multipart, plupload will add the name and chunk params to the URL after any URL you already set for the uploader, for each file, so this is where extra params go.