This is neither a bug report nor a feature enhancement. It's actually request for a new feature in HTTP protocol probably or HTTP feature. In the era of modems, FlashGet was very unique and useful downloader. It sliced an original file to download and each FTP session assigned to each slice was downloaded independently and at the same time. It boosted download speed. In the era of Web, shouldn’t there be a program or HTTP feature like that? Also, Mike Zuckerberg, the CEO of Facebook said that video post will become mainstream soon. However, even the mid service of ISPs like AT&T does’t have sufficient bitrate for live streaming and also for non live, uploading video takes too much time. So, even for ‘Uploading’ side, shouldn’t there be such multi-slice upload feature? Some would say that internet speed will be higher in coming years for the same price, but even though it's true files to download/upload will become even larger, especially we are seeing the wake of 4K video. I know it's overkill for the most of users, but there will be people who will upload/download such video files. If one file transfer session over HTTP doesn't choke up the whole bandwidth of your internet bandwidth, it will be beneficial just like how FlashGet did in the era of telephone modem.
That you for the report! WebKit does not implement http, it relies on platform networking libraries, such as CFNetwork on Mac or libcurl on Linux. If you would like Apple to look into this, please file an enhancement request via https://bugreport.apple.com