TL;DR you can’t.

^{If you’re wondering why this question still hasn’t got an accepted answer, you can read this meta question created by OP, and my answer.}

File drag/drop in HTML5

I made some research in different pieces of documentation for this topic and tested it by myself on various browsers, so I decided to summarize all I know about drag and drop of files here.

Dragging

When you drag a file you can use some listeners, such as:

• dragenter
• dragover
• dragend
• dragleave

Given that these are drag events, the files property of event.dataTransfer will either have length == 0 or be empty (null).

You can’t read files details in a drag event and you can’t check if they are folders. This is not a bug, it’s a security feature.

Imagine you could read files on a drag event: you would be able to read everything even if the user doesn’t want to upload files to your site. It would make no sense, seriously. Imagine you are dragging a file from your desktop to another folder and you accidentally drag it through a web page: now the web page reads your file and stores your personal information on its server… that would be a huge security flaw.

However, you will still be able to detect whether the user is dragging files (and by files I mean folders too, because folders are files) or not by iterating over the array event.dataTransfer.types. You can create a function that checks if the drag event contains files, and then call it in the event handler.

Example:

function containsFiles(event) {
    if (event.dataTransfer.types) {
        for (var i=0; i<event.dataTransfer.types.length; i++) {
            if (event.dataTransfer.types[i] == "Files") {
                return true;
            }
        }
    }
    
    return false;
}

function handleDragEnter(e) {
    e.preventDefault();
    if (containsFiles(e)) {
        // The drag event contains files
        // Do something
    } else {
        // The drag event doesn't contain files
        // Do something else
    }
}

Dropping

When you drop a file into the drop <div> (or whatever element you’re using as dropzone), you will use a listener for the event drop to read some file properties such as name, size, type and last modification date.

To detect if a file is a folder, you are going to:

1. Check if the file has type == "", because folders have no type.
2. Check if the file size is a multiple of 4096: size%4096 == 0, because folders always have a size multiple of 4096 bytes (which is 4KiB).

Example:

function handleDrop(e) {
    e.stopPropagation();
    e.preventDefault();

    var files = e.dataTransfer.files;

    for (var i = 0, f; f = files[i]; i++) { // iterate in the files dropped
        if (!f.type && f.size%4096 == 0) {
            // The file is a folder
            // Do something
        } else {
            // The file is not a folder
            // Do something else
        }
    }
}

KNOWN ISSUE: Since that folders are actually files, this is the only way to distinguish them from another kind of file. Although this method doesn’t give you absolute certainty that a file is a folder: it might be a file without extension and with a size of 0 or exactly N x 4096B.

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Working examples

Here are some working examples to see what I said above in action and test it by yourself. Before running them, make sure that your browser supports drag and drop features. Have fun:

• File drop display info (made by me)
• File/folder recognize (made by me)
• File drag detect (from css-tricks)