attachment filename downloads in non-ascii encodings, ruby, s3

You tell the browser to force a download, and pick a filename for the browser to ‘save as’ with a Content-Disposition header that looks something like this:

Content-Disposition: attachment; filename="filename.tiff"

Depending on the browser, it might open up a ‘Save As’ dialog with that being the default, or might just go ahead and save to your filesystem with that name (Chrome, I think).

If you’re having the user download from S3, you can deliver an S3 pre-signed URL that specifies this header — it can be a different filename than the actual S3 key, and even different for different users, for each pre-signed URL generated.

What if the filename you want is not strictly ascii? You might just stick it in there in UTF-8, and it might work just fine with modern browsers — but I was doing it through the S3 content-disposition download, and it was resulting in S3 delivering an XML error message instead of the file, with the message “Header value cannot be represented using ISO-8859-1.response-content-disposition”.

Indeed, my filename in this case happened to have a Φ (greek phi) in it, and indeed this does not seem to exist as a codepoint in ISO-8859-1 (how do I know? In ruby, try `”Φ”.encode(“ISO-8859-1”)`, which perhaps is the (standard? de facto?) default for HTTP headers, as well as what S3 expects. If it was unicode that could be trans-coded to ISO-8859-1, would S3 have done that for me? Not sure.

But what’s the right way to do this?  Googling/Stack-overlowing around, I got different answers including “There’s no way to do this, HTTP headers have to be ascii (and/or ISO-8859-1)”, “Some modern browsers will be fine if you just deliver UTF-8 and change nothing else” [maybe so, but S3 was not], and a newer form that looks like filename*=UTF-8''#{uri-encoded ut8} [no double quotes allowed, even though they ordinarily are in a content-disposition filename] — but which will break older browsers (maybe just leading to them ignoring the filename rather than actually breaking hard?).

The golden answer appears to be in this stackoverflow answer — you can provide a content-disposition header with both a filename=$ascii_filename (where $filename is ascii or maybe can be ISO-8859-1?), followed by a filename*=UTF-8'' sub-header. And modern browsers will use the UTF-8 one, and older browsers will use the ascii one. At this point, are any of these “older browsers” still relevant? Don’t know, but why not do it right.

Here’s how I do it in ruby, taking input and preparing a) a version that is straight ascii, replacing any non-ascii characters with _, and b) a version that is UTF-8, URI-encoded.

ascii_filename = file_name.encode("US-ASCII", undef: :replace, replace: "_")
utf8_uri_encoded_filename = URI.encode(filename)

something["Content-Disposition"] = "attachment; filename=\"#{ascii_filename}\"; filename*=UTF-8''#{utf8_uri_encoded_filename}"

Seems to work. S3 doesn’t complain. I admit I haven’t actually tested this on an “older browser” (not sure how old one has to go, IE8?), but it does the right thing (include the  “Φ ” in filename) on every modern browser I tested on MacOS, Windows (including IE10 on Windows 7), and Linux.

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