Long File Names With.Spaces In Them

The DCC protocol doesn't take into account the possibility of a filename containing spaces, so most if not all IRC clients will incorrectly interpret the following DCC Send message:

PRIVMSG nick :DCC SEND This is a long file name with.spaces in it ipaddress port filesize

Thus mIRC gives you the Fill Spaces option; this fills spaces in a filename with the underscore character _, which should then allow other clients to interpret the message correctly. So other clients would see:

PRIVMSG nick :DCC SEND This_is_a_long_file_name_with.spaces_in_it ipaddress port filesize

If the Fill Spaces option isn't selected then mIRC sends "Long File Names with.Spaces in them" enclosed in quotes. For example:

PRIVMSG nick :DCC SEND "This is a long file name with.spaces in it" ipaddress port filesize

As far as I know, only mIRC can send and receive messages of this form (and only versions 3.8 and onwards), so if you try using this dcc send message with other clients it probably won't work.