You can append the custom file name to the URL and parse it using packagizer rule.
Rewrite URL in this format:
https://board.jdownloader.org/server-file.mp4#filename=custom-filename.mp4
filename download: custom-filename.mp4
Import this rule in Packagizer (Right-click the 'Raw' button to save file to disk)
https://gist.github.com/rodrigolmacedo/231a7692ace1b0cbf6aa7dc82d0e665a
original post outdated
<jd:append>
-> this placeholder will be replaced by previous value where the placeholder is being used in
<jd:prop:KEY>
-> this placeholder reads out an internal property of the downloadlink itself. properties are plugin specific and there are plans that plugins can return a list of supported properties that can be used here
<jd:indexof>
-> this placeholder will be replaced by number of index of the downloadlink within its package. first link will be 1, second link will be 2 and so on. It's just the index within the package!
-> will be replaced in real time
<jd:subfolderbyplugin>
-> some plugins support recursive folder support. if supported by plugin, then the downloads will have same directory structure like shown in browser
<jd:env:KEY>
-> this placeholder will be replaced by KEY environment variable of your system
-> will be replaced in real time
<jd:orgfilename> <jd:orgfilename:number>
-> this placeholder will be replaced by original filename or matching group(number) of filename pattern
<jd:packagename>
-> this placeholder will be replaced by the current package name
-> will be replaced in real time
<jd:orgfiletype> <jd:orgfiletype:Pattern>
-> this placeholder will be replaced by the file extension of the file or the match of the given pattern
<jd:orgfilenamewithoutext>
-> this placeholder will be replaced by original filename without file extension
<jd:hoster:number>
-> this placeholder will be replaced by the matching group(number) of downloadurl pattern or the host itself if number is -1
<jd:source:(number)pattern>
-> this placeholder will be replaced by the match of sourceurl pattern. with number you can specify which sourceurl to process
<jd:simpledate:simpleDateFormatString>
-> this placeholder will be replaced by time formatted with simpleDateFormatString, see docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html
-> will be replaced in real time
<jd:orgpackagename>
-> this placeholder will be replaced by the original package name ```
https://board.jdownloader.org/showthread.php?p=421993