uploads get renamed to *.dat

[twan]

When adding a file (named test.txt), this file gets subsequently renamed, with each upload to 1.dat, 2.dat, etc.
The content of the uploaded file is intact. It's just this renaming which is puzzling me.
Where is this renaming coming from?

[Stephen]

Just a way of dealing with duplicate filenames.

[twan]

The file I added (test.txt) isn't even there. The file gets the name 1.dat from the start..

[Stephen]

Right, it is saved physically as 1.dat, but when you download it or check it out it will be the original filename.