diff --git a/src/aws/s3.coffee b/src/aws/s3.coffee
index e4e1443..64a7fad 100644
--- a/src/aws/s3.coffee
+++ b/src/aws/s3.coffee
@@ -85,4 +85,9 @@ class S3
       method: 'GET'
       headers: @makeHeaders params
 
+  headObject: (key, params) ->
+    @request @baseURL + key, null,
+      method: 'HEAD'
+      headers: @makeHeaders params
+
 export default S3
\ No newline at end of file
diff --git a/src/index.coffee b/src/index.coffee
index 616cf56..40ea93b 100644
--- a/src/index.coffee
+++ b/src/index.coffee
@@ -1,5 +1,8 @@
 import config from "../config.json"
 import * as crypto from "./crypto"
+import S3 from "./aws/s3"
+
+s3 = new S3 config
 
 addEventListener 'fetch', (event) =>
   event.respondWith handleRequest event
@@ -23,7 +26,22 @@ handlePUT = (request) ->
   unless valid
     return verifyFailure()
 
-  return new Response "Valid"
+  # Don't overwrite existing files
+  resp = await s3.headObject url.pathname[1...], {}
+  if resp.ok
+    return new Response "File already exists",
+      status: 400
+
+  try
+    await s3.putObject url.pathname[1...], request.body,
+      ContentType: request.headers.get "Content-Type"
+      ContentLength: request.headers.get "Content-Length"
+  catch err
+    console.log err
+    return new Response "Cannot upload",
+      status: 400
+
+  return new Response "OK"
 
 verifyFailure = ->
   return new Response "Invalid signature",
diff --git a/webpack.config.js b/webpack.config.js
index ca0ec03..7ad535b 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -9,6 +9,7 @@ module.exports = {
   mode: 'production',
   resolve: {
     extensions: ['.coffee', '.js'],
+    fallback: { "os": false },
   },
   module: {
     rules: [