Download from lightbox fails to authenticate

[gnprice]

Steps to reproduce:

• Log into an org on zulipchat.com, or any other server running a recent release and on Xenial. (Not chat.zulip.org, which is on Trusty.)
• Find a conversation with a user-uploaded image in it. (Or just upload one.)
• Tap the image, bringing up the lightbox.
• Tap the ellipsis in the lower right to bring up a menu; select "Download file".
• The download fails. It should succeed.

I tested this just now with 13.4.88 on Android. I'm pretty sure the cause is that we just haven't fixed the download to provide authentication to the server. For more details, see commit cb31986, where we fixed the display of images within the message list webview.

[gnprice]

Relatedly, a user on iOS reported they were unable to open attachments. This comment is a note to myself to file a proper issue for that, which will probably be a followup to this one.

[EDIT: Filed now, as #2654 .]

[borisyankov]

It seems we are not providing auth on iOS.
The issue though is the redirect. Related #2619

[borisyankov]

Also, the issue could be the auth headers which we can not provide with this API - CameraRoll.saveToCameraRoll accepts a string, not a resource.

[gnprice]

Still repro's on 17.1.98 on Android.

[gnprice]

@jackrzhang FYI: I just filed #3018 discussing why it'd be good to combine our .android.js/.ios.js file pairs. I noticed today there seems to be one of them involved here; in a PR addressing this issue it'd probably be helpful to go ahead and combine that pair, up front at the start of the branch.

[jackrzhang]

A note relevant to Greg's comment above -

7eb97b44 was merged, which combined downloadFile.ios.js and downloadFile.android.js.