Thumbnails for images with spaces in their name do not appear at all. The thumbnail system seems to render the space as a '+' which results in a page not found error, whereas the actual attachments use '%20' which seems to work.

Workaroud:

As workaround users can browse through the attachments section (Tools > Attachments) of the page, and for the attachment you have a space on it (in example: my file.jpg), click in properties and change the file name (so for the my file.jpg you could rename it to myfile.jpg). The image should display fine after that.