Content Server - Books only in RAR/ZIP?

[Starson17]

Moved from Recipe subforum, where I managed to post it without realizing where I was:

I was talking to my Mom via phone as she browsed my Calibre content server from far far away. I happened to mention that there were some short mp3 audio snippets on History that my wife and I found fascinating. Thinking quickly, I compressed them up, dropped them into Calibre as a rar file and suggested she grab them. Calibre refused to send them. I tried to access the content server locally, and it also refused to send me the files. I also tried in zip format. There were 10 mp3s compressed into each.

Is there something in the content server that checks to see if the content of the zip or rar is a book and otherwise refuses to send it?

Adding the mp3s as individual mp3s worked as expected, and she could get them one-by-one.

Thanks for any info or comments!
Kovid asked:

When you say "refused to send them" what exactly do you mean?

I responded:
Sorry, I mean that no file was received. (I seem to have inadvertently posted in the recipe forum - thanks for responding anyway.)

Using Firefox, the ZIP and RAR files can be seen when Details are chosen. Using Save Link As from a right click on the RAR format, the FF Download says that a 0 byte file was received, but there's nothing in the download folder. Doing the same thing with a RAR of a PDF or a TXT format, and Calibre sends a RAR file that opens to see one or more PDFs/TXTs.