Support uploading blobs, with data-filename attribute

Hi Max,

This PR allows someone to upload Blobs as opposed to files.  Blobs don't necessarily have filenames, and the S3Multipart::Upload#validate_filetype expects the upload to have a name, so I added the data-filename attribute on the input file element which is read when the JS upload.name is null.

To upload a blob, set the S3MP fileList property to [myBlob, ...]

If you have a preferred way to handle this, please suggest.   Thanks!

-Michael