Yesterday I advised against using spaces in the names of any files which might end up on the web, including PDF and even Microsoft Word files. While it’s quite possible that they’ll never cause you any problems, I’ve seen many occasions where they have done, and it’s a small concession to make for the avoidance of any grief. On a similar theme, today I’d like to suggest not using upper case letters either. Again, we’re all used to saving files with a mixture of upper and lower case letters on our PCs. But, again, much of the web is based on older technology, and you can never be sure if “BlueWidgets.jpg” will be seen as the same as “bluewidgets.jpg”, never mind whether you want it to be seen as the same in the first place. On our web server, https://www.bmon.co.uk/images/BlueWidgets.jpg will get you to an image, but https://www.bmon.co.uk/images/bluewidgets.jpg won’t.
Don’t make the mistake of thinking “I have a convention, I’ll always use the “BlueWidgets” style for my filenames”, because trust me: many users who type your filenames into their browsers will just use lower case. And can you be sure that’ll find the same file? No.