when I try to upload a large file, localstack basically freezes (and eventually times out).  
But if the file is all `1` bytes it succeeds.

succeeds:
```bash
aws s3 --endpoint-url=http://localhost:4572 mb s3://testbucket
python -c "print '\x01'*10000000" > testfile
echo $(ls -ahl | grep testfile)
aws s3 --endpoint-url=http://localhost:4572 cp testfile s3://testbucket/testfile
```

fails:
```bash
aws s3 --endpoint-url=http://localhost:4572 mb s3://testbucket
python -c "print 'a'*10000000" > testfile
echo $(ls -ahl | grep testfile)
aws s3 --endpoint-url=http://localhost:4572 cp testfile s3://testbucket/testfile
```
so, for me, both the file content and its size determine if this triggers or not. 
I'm using the latest version `0.9.5`
keys = keys if isinstance(keys, list) else [keys]
        objects = [{'Key': k} for k in keys]
        if objects:
            self.s3_client.delete_objects(Bucket=bucket_name, Delete={'Objects': objects})
        self.s3_client.delete_bucket(Bucket=bucket_name)
    def _perform_multipart_upload(self, bucket, key, data=None, zip=False, acl=None):
        kwargs = {'ACL': acl} if acl else {}
        multipart_upload_dict = self.s3_client.create_multipart_upload(Bucket=bucket, Key=key, **kwargs)
        upload_id = multipart_upload_dict['UploadId']
        
        data = data or (5 * short_uid())
        data = to_bytes(data)
        upload_file_object = BytesIO(data)
        if zip:
            upload_file_object = BytesIO()
            with gzip.GzipFile(fileobj=upload_file_object, mode='w') as filestream:
                filestream.write(data)
        response = self.s3_client.upload_part(Bucket=bucket, Key=key,
            Body=upload_file_object, PartNumber=1, UploadId=upload_id)
        multipart_upload_parts = [{'ETag': response['ETag'], 'PartNumber': 1}]
        return self.s3_client.complete_multipart_upload(
            Bucket=bucket, Key=key, MultipartUpload={'Parts': multipart_upload_parts}, UploadId=upload_id
        )
    def _perform_presigned_url_upload(self, bucket, key):
        url = self.s3_client.generate_presigned_url(
            'put_object', Params={'Bucket': bucket, 'Key': key}
        )
        url = url + '&X-Amz-Credential=x&X-Amz-Signature=y'
        requests.put(url, data='something', verify=False)