FineUploader - Error on multi-part upload to S3 - amazon-s3

Question

I am using FineUploader to upload to S3. I have everything working including deletes. However, when I upload larger files that get broken into multi-part uploads, I get the following error in the console (debugging turned on):
Specific problem detected initiating multipart upload request for 0: 'The request signature we calculated does not match the signature you provided. Check your key and signing method.'.
Can someone point me in the right direction as what I should check for settings, or what additional info you might need?

Answer 1

Since you haven't included anything really specific to your setup, code, or the failing request, my best guess is that your server isn't returning a proper signature response for uploads made to the S3 REST API (which is used for larger files). You'll need to review that procedure for generating a response to this type of signature request.
Here's the relevant section from Fine Uploader's S3 documentation:
Fine Uploader S3 uses Amazon S3’s REST API to initiate, upload,
complete, and abort multipart uploads. The REST API handles
authentication by signing canonically formatted headers. This signing
is something you need to implement server-side. All your server needs
to do to authenticate and supported chunked uploads direct to Amazon
S3 is sign a string representing the headers of the request that Fine
Uploader sends to S3. This string is found in the payload of the
signature request:
{ "headers": /* string to sign */ }
The presence of this property indicates to your sever that this is, in
fact, a request to sign a REST/multipart request and not a policy
document.
This signature for the headers string differs slightly from the policy
document signature. You should NOT base64 encode the headers string
before signing it. All you must do, server-side, is generate an HMAC
SHA1 signature of the string using your AWS secret key and then base64
encode the result. Your server should respond with the following in
the body of an ‘application/json’ response:
{ "signature": /* signed headers string */ }