warning filename not safe

In our Dutch site I get a warning message:
Deze filenaam is niet veilig voor gebruik in een URL

This filename is not safe for usage in a url.

When does this message appear and what to do about it.

It's usually when the filename contains any spaces or special characters. It may still work, but it could also break. It depends a bit on operating systems and browsers of the visitors if it works or not.

If you upload the file using SermonSpeaker, then it should sanitise the filename automatically.

I do not use the uploadfunction, perhaps that is a better solution.
I will examine the filename.
Thanks for your quick reply (as usual )

The filename is: 19940830-bijbelstudieavond-PSlagter-tussentijd-01.mp3
I wonder what could be wrong with this name, perhaps too long, or the capital PS?

Yep, looks like the name has to be all lowercased.
SermonSpeaker uses a Joomla function to determine that. It's called `JApplicationHelper::stringURLSafe()`.
If the value returned by that call is different from the value it passed to it, that message will be generated.