Go to https://www.untergrundwelle678.de/ and click on on the download button right to the select-box with "MPEG-1 Audio Layer III". If I look in piwik GUI and there under downloads I only see this:
It seems that the Filename is cut off after the hash.
I'm using Piwik 2.4.1.
Ok, I dug into the code and found out, in short that you cant log files with hashes.
Piwik uses in PageUrl::excludeQueryParametersFromUrl parse_url which is the "culprit", because it thinks that everything after the hash is a fragment, which is perfectly fine, because there is no way to know from a pure URL that this is a file.
I'll close this Ticket and will rename all the files so the downloads can be tracked.