Thank you Jean-François for your reply. I tried your suggestion of leaving the "File name" empty, but it did not change the issue.
I think that those temp files are only temporary files that are supposed to replace the file (with the given name), and each time the temp file has a different name. And actually, those temporary files are replacing the files with right names once (when I had them named "file1", "file2",...), and only after that the files start to accumulate, adding the number to the end of the file name ("file11", "file21",...).
Any ideas for a solution?