The mimetype in question is:
x-scheme-handler/ed2k
I just tried it.
I installed amule and created a file userapp-amule.desktop in ~/.local/share/applications:
[Desktop Entry]
Name=aMule
Name[en_US]=userapp-amule
Exec=amule %u
Icon=amule
Terminal=false
Type=Application
Categories=Network;P2P;
Comment=A client for the eD2k network
MimeType=x-scheme-handler/ed2k
and I also added this line:
x-scheme-handler/ed2k=userapp-amule.desktop
to ~/.local/share/applications/mimeapps.list under [Default Applications] and [Added Associations]. I then opened an ed2k link with xdg-open (note the quotes):
xdg-open 'ed2k://|file|Some.String.Here%5D.%E4%B8%93%E8%BE%91.%28PDF%29.rar|74945828|3788c937a5d436cff6e6808b02d07339|h=4jk63hyesnpxk6xwm5rwgmb2mljxtixz|/'
and it worked:
: Initialising aMule SVN compiled with wxGTK2 v2.8.12 (Snapshot: rev. 10792)
: Checking if there is an instance already running...
: No other instances are running.
: ListenSocket: Ok.
: Loading temp files from /home/me/.aMule/Temp.