POST /camli/preupload HTTP/1.1
Host: example.com

camliversion=1&
blob1=sha1-9b03f7aca1ac60d40b5e570c34f79a3e07c918e8&
blob2=sha1-9242dbadb7827d697fab034a1e73f366b451ce4d

HTTP/1.1 200 OK
Content-Length: ...
Content-Type: text/plain

{
   "maxUploadSize": 1048576,
   "alreadyHave": [
      {"blobRef": "sha1-9242dbadb7827d697fab034a1e73f366b451ce4d",
       "size": 12312}
   ],
   "uploadUrl": "http://example.com/SDALKSJDALKSJDKLASJD",
   "uploadUrlExpirationSeconds": 7200,
}




POST /SDALKSJDALKSJDKLASJD HTTP/1.1
Host: example.com
Content-Type: multipart/form-data; boundary=----------------------------9782c16cfc60

----------------------------9782c16cfc60
Content-Disposition: form-data; name="sha1-9b03f7aca1ac60d40b5e570c34f79a3e07c918e8"
Content-Type: application/octet-stream

(binary blob data)
----------------------------9782c16cfc60
Content-Disposition: form-data; name="sha1-9242dbadb7827d697fab034a1e73f366b451ce4d"
Content-Type: application/octet-stream

(binary blob data)
----------------------------9782c16cfc60--

HTTP/1.1 200 OK
Content-Type: text/plain

{
   "maxUploadSize": 1048576,
   "uploadUrl": "http://example.com/TheNextUploadUrlRandomString",
   "uploadUrlExpirationSeconds": 7200,
}

If connection drops during a POST to an upload URL, you should re-do a
preupload request to verify which objects were received by the server
and which were not.  Also, the URL you received from preupload
might've been bogus.


TODO: resuming uploading large blobs on slow crappy phone links