Add a File (JS object) to input type="file"

Question

I need a way to add a File object to a file input field.

Note that I do not want to set the value of the file field. (which is a security risk, ... I know that)
I already have the file itself (and all its contents!)

I'm creating the File object like this:

canvas.toBlob(function(blob) {
  var file = new File([blob], 'file.png');
  // and now I want to add MY File to the input here
});

Please don't tell me about how to upload the File, I know about XMLHttpRequest Level 2. I want to upload the File to an external website (potentially using a userscript or something similar).

So, how can I achieve that? Or if it's not possible: why can't I add a File that I actually own (because I created it in the Browser (as a virtual File, which does not even exist in the users filesystem)) to an input field that I own too? (technically the input field could be my own, that does not matter here)

Solution

No, you can't.

Taken from here:

It seems like you want to take the File object from the drop event and assign it to the element. Unfortunately, you can't do that. Only the user can select files; you can't dynamically change the files which will be uploaded because browsers deny JavaScript this ability for security reasons.