Steps to Reproduce

Filedownload.save("test content", "text/plain", "test#test.txt");

Current Result

will download the file as test.txt omitting #test

Expected Result

file downloaded as test#test.txt

Debug Info

in DesktopImpl.getDownloadMediaURI special characters inside the file name like "#?&" need to be %-encoded to avoid file name truncation
see: https://en.wikipedia.org/wiki/Percent-encoding#Percent-encoding_reserved_characters

Workaround

split download code such as ...

Filedownload.save("test content", "text/plain", "test#test.txt");

... into ...

AMedia media = new AMedia("test#test.txt", "txt", "text/plain", "test content");
Clients.response(new AuDownload((DeferredValue) () -> Executions.getCurrent().getDesktop().getDownloadMediaURI(media, "test#test.txt").replace("#", "%23")));

... allowing to encode special chars as needed

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Alternative is to override the getDownloadMediaURI method by extending DesktopImpl and configure a custom webapp factory to return instances of the custom DesktopImpl class