heres how to actually test if your router is altering the message.
Start 2 copies of the client up and attach to the same irc server (it can be done with one but i dont care!)
On both copies dont be in any channels (this is to reduce the clutter of stuff you well be looking at)
On both copies type in /DEBUG @DEBUG
(this window well show you all traffic sent to and from the client to the server)
now on copy ONE attempt to DCC SEND to the second copy
On client ONE you well see a message with this text in it "DCC SEND <filename> <number> <number> <number>"
On client TWO you should normally see the same thing coming in, if the router has altered it the middle number well have changed, if the router altered it real badly you might have anything after the filename.
* do not confuse this with the NOTICE message mirc also sends (which normally well be sent fine regardless)