Handling File Uploads with S3 using Pre-signed URLs

If you’re looking to limit file uploads to a specific size when using pre-signed URLs with S3, you’re on the right path thinking about headers and checks!

Restricting File Size with Pre-signed URLs

Unfortunately, you can’t directly set a file size limit in the pre-signed URL itself. However, you can enforce a maximum file size by including the x-amz-content-sha256 header in your request and validating the size on your server.

How to Check File Size

Here’s a basic approach you could consider for implementing the size limit:

• On the client side, use JavaScript to check the file size before the upload:

if (file.size > 5 * 1024 * 1024) { // 5MB
    alert('File size exceeds 5MB!');
    return;
}

• When generating the pre-signed URL, you can add restrictions, but you’ll have to implement checks on the server-side after the upload attempt.
• For server-side checks, inspect the Content-Length header in the request after receiving it.

User Notifications

Providing user feedback can really help. Here are some tips:

• Prompt Users: Make sure to inform users about the size limit before they start uploading. A simple message or tooltip can do wonders!
• Visual Cue: You can use a file size indicator. For example, show the maximum size allowed next to the upload button.
• Preview Feature: Give users a preview of the file they’re about to upload along with its size.

Example Code

Here’s a pseudo example of what your JavaScript might look like:

document.getElementById('fileInput').addEventListener('change', function(event) {
    const file = event.target.files[0];
    if (file && file.size > 5 * 1024 * 1024) {
        alert('File size exceeds 5MB. Please choose a smaller file.');
        event.target.value = ''; // Clear the input
    }
});

By doing this, you’ll save yourself and your users a lot of hassle!