The server is not returning anything useful in the HTTP headers (I wouldn't expect it to):
~/tmp$ wget -S 'http://u801.wapkafile.com//g03/video/1253022/7940/c8f9a32ef15648bfa6f693102de27835/DARNA-ZAROORI-HAI-3(Movies7.In).avi?md5=TU7ibYa85byjzJyJcH_LXQ&expires=1458916610'
--2016-07-27 18:21:15-- http://u801.wapkafile.com//g03/video/1253022/7940/c8f9a32ef15648bfa6f693102de27835/DARNA-ZAROORI-HAI-3(Movies7.In).avi?md5=TU7ibYa85byjzJyJcH_LXQ&expires=1458916610
Resolving u801.wapkafile.com (u801.wapkafile.com)... 8.37.229.38
Connecting to u801.wapkafile.com (u801.wapkafile.com)|8.37.229.38|:80... connected.
HTTP request sent, awaiting response...
HTTP/1.1 200 OK
Server: nginx
Date: Wed, 27 Jul 2016 08:21:15 GMT
Content-Type: video/x-msvideo
Content-Length: 65707724
Connection: keep-alive
Last-Modified: Wed, 31 Dec 2014 06:15:08 GMT
Content-Disposition: attachment; filename="DARNA-ZAROORI-HAI-3(Movies7.In).avi"
Accept-Ranges: bytes
Expires: Wed, 03 Aug 2016 06:34:10 GMT
Cache-Control: max-age=604800
Cache-Control: s-maxage=604800,max-age=604800
Age: 6425
X-Cache: HIT TCP_MEM_HIT dirn:0:1402323922
X-Swift-SaveTime: Wed, 27 Jul 2016 06:34:10 GMT
X-Swift-CacheTime: 604800
Via: 440d210b[0,206-0,H]
Length: 65707724 (63M) [video/x-msvideo]
Saving to: 'DARNA-ZAROORI-HAI-3(Movies7.In).avi?md5=TU7ibYa85byjzJyJcH_LXQ&expires=1458916610.1’
So Download Manager must be getting the info from the metadata at the start of the file.
For example, I only downloaded the first 70KB of the file, and:
~/tmp$ file DARNA-ZAROORI-HAI-3\(Movies7.In\).avi\?md5=TU7ibYa85byjzJyJcH_LXQ\&expires=1458916610
DARNA-ZAROORI-HAI-3(Movies7.In).avi?md5=TU7ibYa85byjzJyJcH_LXQ&expires=1458916610: RIFF (little-endian) data, AVI, 640 x 360, ~30 fps, video: H.264 X.264 or H.264, audio: MPEG-1 Layer 3 (stereo, 22050 Hz)
So the info is there at the start. I expect you'll want to use a library to parse it out. I'm not sure if Qt has such functionality built-in. Perhaps have a look through the Qt Multimedia docs.
Cheers.