Helper to enable client-side signing for AWS S3 uploading for Uppy with AWS S3 Multipart. Set callbacks for AWS S3 Multipart plugin to methods from this heper (https://uppy.io/docs/aws-s3-multipart/#createMultipartUpload-file).

aws_s3_multipart_helper.js

import AWS from 'aws-sdk';

export default class AwsS3MultipartHelper {
  constructor(options) {
    this.options = {
      bucket: options.bucket,
      acl: options.acl || 'public-read',
      expires: options.expires || 5 * 60,
    };

    AWS.config.credentials = new AWS.Credentials({
      accessKeyId: options.credentials.accessKeyId,
      secretAccessKey: options.credentials.secretAccessKey,
      sessionToken: options.credentials.sessionToken,
    });

    this.s3 = new AWS.S3({});
  }

  createMultipartUpload(file) {
    return new Promise((resolve, reject) => {
      const metadata = {};
      Object.keys(file.meta).map(key => {
        if (file.meta[key] != null) {
          metadata[key] = file.meta[key].toString()
        }
      })

      const key = file.name;
      
      this.s3.createMultipartUpload({
        Bucket: this.options.bucket,
        Key: key,
        ACL: this.options.acl,
        ContentType: file.type,
        Metadata: metadata,
        Expires: this.options.expires,
      }, (error, data) => {
        if (error) {
          reject(error);
        } else {
          resolve({
            key: data.Key,
            uploadId: data.UploadId,
          });
        }
      });
    });
  }
  
  listParts(file, { key, uploadId }) {
    return new Promise((resolve, reject) => {
      let parts = [];
      
      const listPartsPart = (part) => {
        this.s3.listParts({
          Bucket: this.options.bucket,
          Key: key,
          UploadId: uploadId,
          PartNumberMarker: part,
        }, (error, data) => {
          if (error) {
            reject(error);
            return;
          }

          parts = parts.concat(data.Parts);
          if (data.IsTruncated) {
            listPartsPart(data.NextPartNumberMarker);
          } else {
            resolve(parts);
          }
        })
      }

      listPartsPart(0);
    });
  }
  
  prepareUploadPart(file, { key, uploadId, number }) {
    return new Promise((resolve, reject) => {
      this.s3.getSignedUrl('uploadPart', {
        Bucket: this.options.bucket,
        Key: key,
        UploadId: uploadId,
        PartNumber: number,
        Body: '',
        Expires: this.options.expires,
      }, (error, url) => {
        if (error) {
          reject(error);
        } else {
          resolve({ url });
        }
      });
    });
  }
  
  abortMultipartUpload(file, { key, uploadId }) {
    return new Promise((resolve, reject) => {
      this.s3.abortMultipartUpload({
        Bucket: this.options.bucket,
        Key: key,
        UploadId: uploadId
      }, (error, data) => {
        if (error) {
          reject(error);
        } else {
          resolve({});
        }
      });
    });
  }
  
  completeMultipartUpload(file, { key, uploadId, parts }) {
    return new Promise((resolve, reject) => {
      this.s3.completeMultipartUpload({
        Bucket: this.options.bucket,
        Key: key,
        UploadId: uploadId,
        MultipartUpload: {
          Parts: parts
        }
      }, (error, data) => {
        if (error) {
          reject(error);
        } else {
          resolve({ location: data.Location });
        }
      });
    });
  }
};

I setup this helper and Uppy like this:

/*
 * `options` is generated by our server. It contains information about upload and credentials to use. It is expected to look like this:
 * {
 *   bucket: 'bucket', // name of bucket you want to upload to
 *   acl: 'public-read, // ACL you want to set for uploaded objects
 *   credentails: { // actual AWS credentials you want to use, we generate them using AWS STS GetFederationToken (https://docs.aws.amazon.com/STS/latest/APIReference/API_GetFederationToken.html)
 *     accessKeyId: '',
 *     secretAccessKey: '',
 *     sessionToken: ''
 *   }
 * }
 */
const options = { ... };

const awsS3MultipartHelper = new AwsS3MultipartHelper({
  bucket: options.bucket,
  acl: options.acl,
  credentials: options.credentials,
});

const uppyOptions = {};

const awsS3MultipartOptions = {
  limit: options.limit || 5,
  
  createMultipartUpload: (file) => awsS3MultipartHelper.createMultipartUpload(file),
  listParts: (file, options) => awsS3MultipartHelper.listParts(file, options),
  prepareUploadPart: (file, options) => awsS3MultipartHelper.prepareUploadPart(file, options),
  abortMultipartUpload: (file, options) => awsS3MultipartHelper.abortMultipartUpload(file, options),
  completeMultipartUpload: (file, options) => awsS3MultipartHelper.completeMultipartUpload(file, options),
}

const uppy = Uppy(uppyOptions).use(AwsS3Multipart, awsS3MultipartOptions);

Now you can use Uppy as usual, it will use the helper to sign AWS requests on client side.