What Happens When You Upload a Large File & Generate a Temporary Download Link

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Reetain Raina Sending a 5 GB video file might seem simple. You select the file, click Upload, wait for the progress bar to finish, and share a link with someone. But there are several things happening in the background while the file is being uploaded. The file has to travel across a network, reach a server, and be stored somewhere before another person can download it. The service also needs to make sure that the recipient can access the file without accessing the sender's account. If the internet connection drops halfway through the upload, it needs a way to continue without sending the entire file again. File transfer services use HTTP requests, storage systems, databases, encryption, and background processes to manage these tasks. But how does a website actually handle a file that's several gigabytes in size?

Does it upload everything in one request?

Where does the file go after the upload finishes?

And what happens when its download link expires?

In this article, we'll look at how file transfer services handle large uploads, store files, generate download links, and manage interrupted connections and expired transfers. Then, once you understand the theory behind how it all works, we'll also build a basic chunked file uploader using Node.js to see some of these processes in action. What We'll Cover: Why Email Attachments Fail: SMTP Wasn't Designed for Large Files SMTP and Attachment Size Limits The Base64 Problem The Core Architecture: What Happens Behind a File Transfer Service?

The Client The Upload Server The Storage Layer The Metadata Database The Download Server Chunked and Resumable Uploads: How Large Files Survive Bad Internet Connections Breaking a Large File Into Smaller Chunks What Happens When the Internet Disconnects?

Checksums: Making Sure Nothing Gets Corrupted Storage and Link Generation: Where Does Your File Actually Go?

Why Services Use Object Storage How a Download Link Is Created Why Are These Links Difficult to Guess?

Expiry and Cleanup: What Happens When a Transfer Expires?

Time to Live (TTL) Automated Cleanup Jobs Download Limits Security Layers: Protecting Files During Transfer and Storage TLS Encryption in Transit Encryption at Rest Password-Protected Links Real-World Examples: How Popular File Transfer Services Handle File Sharing WeTransfer: Temporary Transfers and Password Protection Dropbox Transfer: Sending Files Without Collaboration Send Anywhere: Direct Transfers and Temporary Links Filemail: Large Transfers and Expiration Controls Hands-On: Build a Minimal Chunked File Uploader Using Node.js Step 1: Set Up the Node.js Project Step 2: Divide the File Into Smaller Chunks Step 3: Create an Upload Endpoint in Express Step 4: Reassemble the Original File Step 5: Test the Upload Conclusion: A Simple Link Hides a Lot of Engineering Why Email Attachments Fail: SMTP Wasn't Designed for Large Files Before looking at dedicated file transfer services, it helps to understand why we can't just attach massive files to an email. Sending large files through email is fundamentally flawed because the underlying architecture was built for text, not high-resolution media. SMTP and Attachment Size Limits Simple Mail Transfer Protocol (SMTP) handles the transmission of email messages across the internet. Because servers have limited bandwidth and storage, email providers impose strict attachment size limits, usually capping out around 25 MB. However, these limits apply to the encoded message size, not just the original file sitting on your hard drive. The Base64 Problem Email attachments are commonly encoded using MIME and Base64 formats. Base64 is necessary because it converts binary data into text-compatible characters that email servers can safely route. While this solves the transmission problem, it creates a massive efficiency issue: this encoding increases the data size by approximately 33%. You can read the foundational specifications for this in RFC 2045, which defines MIME's Base64 transfer encoding. In