DotNetZip – Solving Mac Decompression Issues

As part of a ASP.Net Web API service that I’m co-developing, I have gotten familiar with the DotNetZip Library.  It’s fantastic.  We’re using it to gather up a set of requested files from network storage, package them in a zip archive, and stream them back to a client via a web application.

During testing, things worked great on Windows machines, but the moment we tested using a Mac, we ran into issues.


Problem:  On a Mac, a zip file would be downloaded, but it could not be extracted.  Double-clicking the zip file resulted in the creation of a .cpgz file.

Solution:  In ASP.Net code, change

HttpContext.Current.Response.ContentType = “application/zip”


HttpContext.Current.Response.ContentType = “application/octet-stream”

Problem Details:

A zip archive would download as normal, but as things were wrapping up, Safari would try to decompress the file and it would fail.

Error from Safari’s download manager area:

Furthermore, when attempting to simply extract the files directly using the built in Mac Archive Utility (by double-clicking the zip file), the utility would actually re-archive the file, rather than extract it.  The end result was the creation of a .cpgz file.  Attempting to decompress that file would result in creating another .zip file, which, when double-clicked, created another .cpgz file, and so on, and so on.

Archive Utility re-archiving the zip file when double-clicked (rather than un-archiving it)

Screen Shot 2013-10-10 at 4.13.48 PM

Creation of .cpgz file:


Needless to say, the solution to this problem, though somewhat mysterious, was quite simple for me.

In my ASP.Net code, I was writing the zip file to the Response OutputStream.  I had added a content type of “application/zip” to the response.  This is what was breaking the zip files on a Mac.  Buried in the DotNetZip Library’s forum was a post recommending changing the content type to “application/octet-stream” instead.  That single changed fixed the issue for standard zip files!

Solution Details:


HttpContext.Current.Response.ContentType = “application/zip”


HttpContext.Current.Response.ContentType = “application/octet-stream”

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