Upload a File in a Google Chrome Extension

I'm writing an extension for Chrome, and I need to upload a file from the page the user is currently on to my server to be processed, I cannot figure out how to upload the file though. I considered just passing the link to the server and having the server download the file, however if the site requires authentication this will not work. Is it possible to upload a file via a Chrome extension to my server?

Solution 1:

I've recently developed a Chrome extension which retrieves content from a page, and sends it to the server.

The following approach was used:

1. File downloads: Get the src property of an <img> element, for example.
2. Fetch the file from the Cache - use XMLHttpRequest from the background page.
3. Use a Web Worker in the background page to handle the upload.

Side note, to take the checksum of the image, Crypto-JS: MD5 can be used. Example (where xhr is the XMLHttpRequest object with responseType set to arraybuffer, see Worker demo):

var md5sum = Crypto.MD5( new Uint8Array(xhr.response) );

Full example

Content script

// Example: Grab the first <img> from the document if it exists.
var img = document.images[0];
if (img) {
    // Send the target of the image:
    chrome.runtime.sendMessage({method: 'postUrl', url: img.src});
}

Background script (with Worker)

chrome.runtime.onMessage.addListener(function(request) {
    if (request.method == 'postUrl') {
        var worker = new Worker('worker.js');
        worker.postMessage(request.url);
    }
});

Web Worker

// Define the FormData object for the Web worker:
importScripts('xhr2-FormData.js')

// Note: In a Web worker, the global object is called "self" instead of "window"
self.onmessage = function(event) {
    var resourceUrl = event.data;   // From the background page
    var xhr = new XMLHttpRequest();
    xhr.open('GET', resourceUrl, true);

    // Response type arraybuffer - XMLHttpRequest 2
    xhr.responseType = 'arraybuffer';
    xhr.onload = function(e) {
        if (xhr.status == 200) {
            nextStep(xhr.response);
        }
    };
    xhr.send();
};
function nextStep(arrayBuffer) {
    var xhr = new XMLHttpRequest();
    // Using FormData polyfill for Web workers!
    var fd = new FormData();
    fd.append('server-method', 'upload');

    // The native FormData.append method ONLY takes Blobs, Files or strings
    // The FormData for Web workers polyfill can also deal with array buffers
    fd.append('file', arrayBuffer);

    xhr.open('POST', 'http://YOUR.DOMAIN.HERE/posturl.php', true);

    // Transmit the form to the server
    xhr.send(fd);
};

FormData for Web workers POLYFILL

Web workers do not natively support the FormData object, used to transmit multipart/form-data forms. That's why I've written a polyfill for it. This code has to be included in the Web worker, using importScripts('xhr2-FormData.js').

The source code of the polyfill is available at https://gist.github.com/Rob--W/8b5adedd84c0d36aba64

Manifest file:

{
  "name": "Rob W - Demo: Scraping images and posting data",
  "version": "1.0",
  "manifest_version": 2,
  "content_scripts": [
    {
      "matches": ["http://*/*", "https://*/*"],
      "js": ["contentscript.js"]
    }
   ],
   "background": {
       "scripts": ["background.js"]
   },
   "permissions": ["http://*/*", "https://*/*"]
}

Relevant documentation

• Message passing ^{Google Chrome Extensions}
• chrome.runtime.onMessage ^{Google Chrome Extensions}
• XMLHttpRequest Level 2 ^{W3c specification}
• FormData (XHR2) ^MDN
• ArrayBuffer responses (XHR2) ^{HTML5 rocks} (note: arraybuffer responses are deprecated in favor of typed arrays, the polyfill has been updated to reflect this change)

Solution 2:

The simplest solutions seems to be for your extension to send the file's URI to your server, and then your server-side code will download it from the page into the server and process it.

Create a server-side script like http://mysite.com/process.php?uri=[file's URI goes here] that will process the given file. Use AJAX to call this URL (more info at http://code.google.com/chrome/extensions/xhr.html ). The script will return the processed file, which you could then use in your extension.