We had some problems with our SharePoint (on SBS2008) with Zip files. Users could upload the files fine, but when trying to access them it would say:
Cannot open file: it does not appear to be a valid archive. If you downloaded this file, try downloading the file again.
We found the problem was that IIS had compression turned on and was gzipping the Zip file a second time. Following the instructions at http://ziprepairsoftware.blogspot.com/2008/02/problems-with-zip-compression-in-iis.html we solved it by changing the MIME type of Zip to application/octet-stream. The steps were as follows:
- Click on Start, point to Administrative Tools, and then click on Internet Information Services (IIS) Manager.
- Right-click on the server (local computer) tree, where server (local computer) is the name of your IIS server, and then click on Properties.
- Click on MIME Types.
- Scroll down to the registered MIME type list to find the .zip entry.
- Click on the .zip application/x-zip-compressed entry, and then click Edit.
- Modify the MIME type value to application/octet-stream.
- Then Stop and then restart the IIS Admin Service to apply the change to the MIME type.