See if they support range requests

[tj]

could actually seek

[jpillora]

Dropbox public folders support range requests:

$ curl -I https://dl.dropboxusercontent.com/u/14376395/Pictures/apple.png
HTTP/1.1 200 OK
Server: nginx
Date: Mon, 18 Jul 2016 00:44:53 GMT
Content-Type: image/png
Content-Length: 4857
Connection: keep-alive
x-robots-tag: noindex, nofollow, noimageindex
content-disposition: inline; filename="apple.png"; filename*=UTF-8''apple.png
accept-ranges: bytes
content-security-policy: referrer no-referrer
etag: 5995n
pragma: public
cache-control: max-age=0
x-content-security-policy: referrer no-referrer
x-webkit-csp: referrer no-referrer
X-Server-Response-Time: 99