When I try to add a file (.mp3, .ogg, .jpg etc.), I click "Browse file" and get a pop-up message saying "Server error in '/' application. Runtime Error
Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine."

I have added files before with no problem; this just occurred today. I am editing my game completely online, if that helps.

I can't see any error in the logs. It may be that the files you're trying to upload are too large - how big are they, and does it work if you try resizing the images to be smaller?

Why are you uploading .ogg files? They're only supported in 5.6 beta if you're tinkering with html5 afaik. Does net framework support .ogg then, Alex?

Quest only officially supports wav or mp3. I don't think you can use .ogg with the gui way of playing sounds but I could be wrong.

Regardless of whether or not .ogg files are uploadable, the problem remains that I can't upload anything, the popup for uploading files doesn't even come up. Instead I get a popup that says "Server error in '/' application" etc., as I posted before. It doesn't matter how small the file is, it doesn't matter whether it's a .mp3 or a .wav or a .wtf or .fml. I. Can't. Upload. Anything.

So... any help with that?

What browser are you using? Can you post a screenshot of the error?