I'm only asking because I tried sending an email (with PEAR) which containd an image whose name was something like "header image_3021" and noticed that it didn't show up in the email.
When I checked the SRC in the recieved email the space was replaced with + and that somehow made the link point to the wrong file. Now, IIRC, + is a correct encoding for spaces in URLs yet the browser could not locate "header+image_2031".
I checked the original content of the email both with Gmail's show original and in the server logs and the space was still there, so the replacement was done either by the browser or by Gmail's rendering process.
I have since modified my upload algorithms to not allow spaces in filenames but I have to ask: What's the best way to make sure the browser will display images with spaces in their file names? Replace them with %20 myself? Let the browser do it? Just disallow them?