Handle non-ASCII filenames in XHR uploading

Thanks so much for Boris's help. Here's a summary of what I discovered through our interactions in comments:

1) The core issue is that HTTP Request headers are supposed to be ISO-8859-1. Prior versions of Chrome and Firefox both passed along UTF-8 strings unchanged in setRequestHeader() calls. This changed in FF24.0 (and apparently will be changing in Chrome soon too), such that FF drops high bytes and passes along only the low byte for each character. In the example I gave in the question, this was recoverable, but characters with higher codes could be mangled irretrievably.

2) One workaround would be to encode on the client side, e.g.:

setRequestHeader('X-File-Name',encodeURIComponent(filename))

and then decode on the server side, e.g. in PHP:

$filename=rawurldecode($_SERVER['HTTP_X_FILE_NAME'])

3) Note that this is only problematic because my ajax file upload approach is to send the raw file data in the request body, so I need to send the filename via a custom request header (as shown in many tutorials online). If I used FormData instead, I wouldn't have to worry about this. I believe if you want solid, standards-based unicode filename support, you should use FormData and not the request header approach.