Provide option to upload files synchronously, to conserve ordering

[alpeb]

When uploading multiple files they're sent in parallel, which is great for swiftness, but they usually end up in a different order than they were declared in the files property. In our case we're uploading files along with their checksums in separate files, and we'd like the files to be listed next to their checksums. How about providing an option to do the upload synchronously? Thanks!