>>/1623/
Servers typically give that filename in the 'http header' of a file download. Hydrus generally does not parse this in the same way that your browser would and does not push it into the general metadata pipeline of a file import, so I'm afraid the only way you can figure this out is to parse it from the html file or similar, as you would for other metadata like tags or related URLs.
In general, also, most downloader makers do not parse the filename. The imageboard watchers like the 4chan parser does parse for the filename, and gives it a 'filename:' namespace. By default, I have these set not to add in the 'tag import options' for watchers, since most users don't want them.
If you feel clever and brave, you can try to add a 'content parser' to a downloader you are interested in to try and grab filename from the boorus you like, but beyond that, I'm sorry to say I don't have a good answer for you. I've thought a few times about making a 'filename' tag service that remembers hard drive import filenames, and could potentially get server-set filenames too, but every time I return to the idea, I realise it'll probably just get overwhelmed by 'Image.jpg' garbage that isn't typically useful.
Although I can't give you a nice answer, you might like to read my FAQs on this question, just so you see better where I am coming from and why I don't want to put much time into this:
https://hydrusnetwork.github.io/hydrus/faq.html#filenames
https://hydrusnetwork.github.io/hydrus/faq.html#external_files (somewhat related)
And if you would like to dabble with making/editing your own downloaders, check out the help here:
https://hydrusnetwork.github.io/hydrus/downloader_intro.html