How do I remove a file from the FileList

If you want to delete only several of the selected files: you can't. The File API Working Draft you linked to contains a note:

The HTMLInputElement interface [HTML5] has a readonly FileList attribute, […]
[emphasis mine]

Reading a bit of the HTML 5 Working Draft, I came across the Common input element APIs. It appears you can delete the entire file list by setting the value property of the input object to an empty string, like:

document.getElementById('multifile').value = "";

BTW, the article Using files from web applications might also be of interest.

Since JavaScript FileList is readonly and cannot be manipulated directly,

BEST METHOD

You will have to loop through the input.files while comparing it with the index of the file you want to remove. At the same time, you will use new DataTransfer() to set a new list of files excluding the file you want to remove from the file list.

With this approach, the value of the input.files itself is changed.

removeFileFromFileList(index) {
  const dt = new DataTransfer()
  const input = document.getElementById('files')
  const { files } = input
  
  for (let i = 0; i < files.length; i++) {
    const file = files[i]
    if (index !== i)
      dt.items.add(file) // here you exclude the file. thus removing it.
  }
  
  input.files = dt.files // Assign the updates list
}

ALTERNATIVE METHOD

Another simple method is to convert the FileList into an array and then splice it.

But this approach will not change the input.files

const input = document.getElementById('files')
// as an array, u have more freedom to transform the file list using array functions.
const fileListArr = Array.from(input.files)
fileListArr.splice(index, 1) // here u remove the file
console.log(fileListArr)

This question has already been marked answered, but I'd like to share some information that might help others with using FileList.

It would be convenient to treat a FileList as an array, but methods like sort, shift, pop, and slice don't work. As others have suggested, you can copy the FileList to an array. However, rather than using a loop, there's a simple one line solution to handle this conversion.

 // fileDialog.files is a FileList 

 var fileBuffer=[];

 // append the file list to an array
 Array.prototype.push.apply( fileBuffer, fileDialog.files ); // <-- here

 // And now you may manipulated the result as required

 // shift an item off the array
 var file = fileBuffer.shift(0,1);  // <-- works as expected
 console.info( file.name + ", " + file.size + ", " + file.type );

 // sort files by size
 fileBuffer.sort(function(a,b) {
    return a.size > b.size ? 1 : a.size < b.size ? -1 : 0;
 });

Tested OK in FF, Chrome, and IE10+