The "+" sign tells FileMaker, "Insert a space in the filename here." It's part of FileMaker's URL encoding scheme; has been for a while. You can see it if you tell the desktop client to send an email link; it'll show up wherever there's a space in the filename. I don't know that there's any way to tell it to do otherwise.
Someone else may know of a way to encode it so FileMaker interprets it as part of the filename instead, but I don't. Sorry.