No, you cannot use a tag URL with a file extension - unless you want the extension included in all of your tags. Explanation from Brian Suda on the microformats-discuss list:
--- the spec says: ... the last segment of the path portion of the URI (after the final "/" character) contains the tag value.
What that string is, is irrelevant. It could be /file/my.pdf and the tag would be "my.pdf" it could be /downloads/my.big.file.pdf and the tag would be "my.big.file.pdf". A tag is everything after the last slash (unless the last char is a slash). So if that is a file or a folder, or a file that does not have an extention, or a folder that has a '.' in it's name.