The rule of thumb is do not use spaces or plusses "+" in filenames. If you have to, use underscores.
In detail: if you use spaces, Domino converts them to plusses.
eg: "My Profile.doc" will be stored in URLs as "My+Profile.doc" .
If the user then edits the document using word, and saves the document it will be saved as "My+Profile.doc" and if the document is then reattached to a docinon document the confusion will start because domino will try and launch the attached document "My Profile.doc".
If you actually change the plus in the url to be %2B it will work ok.
So basically don't use plusses or spaces in filenames.
It might be worth putting a warning note next to the File upload buttons on the web page if this happens a lot, or writing code to test this at thr front end. May be I will get a round to this one day.
NB Translating spaces to plusses is an old HTML pracice, most applications translate spaces to %20 now. See http://www.notesninjas.com/#CharsInURL for details.