How do I submit a "file" input without submit button using JavaScript?

yes, you can use the form.submit() function. Add an onchange listener on the file input and link it to the form.submit() function, like so:

<form action="upload.php" method="post" enctype="multipart/form-data">
<input type="file" onchange="this.form.submit()" name="myFile"/>
</form>

Yes, you can add the following to the onchange event of the file input:

<input type='file' .... onchange='this.form.submit();'>

this submits the form right after the user has picked a file. However, the user can't correct a mistaken selection before submitting - be sure to check whether this is really wise.

I don't believe you can do this. Browsers are very, very strict about what you can do to file upload fields, because of the potential for abuse. If the user accidentally selects a private file, they wouldn't want it to immediately start uploading that file to a random server.

I'm not sure what the restrictions are with doing this in an HTML form.

You can, however, do it with Flash. Gmail does it - when I click "Attach a file", I get prompted with a file browse dialog, and when I OK that dialog the upload begins immediately, and also gives me a progress bar.

Googling for "Flash uploader" will give you many options, but I don't have experience with any of them.

The solutions that were here to add an event listener in the input didn't work for me, so I found another solution, and I wanted to share it.

HTML:

    <form id="attachUpload" method="POST"  action="javascript:void(0)" accept-charset="utf-8" enctype="multipart/form-data">
<input id="uploadAttachment" type="file" name="files[]" placeholder="Computer" multiple />
</form>

The code to submit the form:

$(document).on('change', '#uploadAttachment', function() {
$('#attachUpload').trigger('submit');

});

And than if you want to post that data after the form is submitted:

$(document).on('submit', '#attachUpload', function(e) {
// code here
})