Meanwhile I studied what is the difference between the same file downloaded with the two versions of zipdownload. Files downloaded with the new plugin is bigger with 2bytes than the ones downloaded with the older plugin. These bytes are inserted to the beginning of the file and are (in hexa): "20 0A". If I manually delete these bytes the file can be viewed just fine by any viewer that support that type of file.
Maybe somewhere in the source code of the new plugin (or in other place of RC) is inserted a "space character + new line".
I also tried to download all attachments in a .zip file and could not unzip that archive. The error displayed by my unzip utility is something like that:
Archive: file.zip
End-of-central-directory signature not found. Either this file is not
a zipfile, or it constitutes one disk of a multi-part archive. In the
latter case the central directory and zipfile comment will be found on
the last disk(s) of this archive.
unzip: cannot find zipfile directory in one of file.zip or
file.zip.zip, and cannot find file.zip.ZIP, period.
When I displayed the content of the file.zip in hexa, the same 2bytes are beeing inserted in the beginning of the file.zip and maybe before each file in the archive.
Maybe the archive file.zip is bigger with (2 bytes * nr_of_files_in_the archive) than it should be.