The upgrade.tgz file for 4.1.2 is gzipped twice. If you unzip upgrade.tgz, you will get upgrade.tar, but this file is actually a gzipped file as well.
I have noticed quite a few *.tgz files from this site suffer from the same problem.
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TWiki:Main/WalterFergusson - 08 Mar 2007
Not a single tgz is zipped twice. It is the twiki.org server which obviously still has problems with the mime types. Let me guess - you downloaded using Internet Explorter?
Michael Daum. You probably need to revisit this again.
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TWiki:Main.KennethLavrsen - 08 Mar 2007
I can confirm the issue. I just tried to download a plugin using Internet Explorer.
TWiki:Plugins.TopicCreatePlugin. I downloaded the zip file and it is molested. I then download with Firefix and it works fine. The download is done from the attachment table - ie from viewfile.
I thought that problem had been fixed once.
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TWiki:Main.KennethLavrsen - 08 Mar 2007
This is much worse than I thought. I just tried to download TWiki-4.1.2 as tgz and I even download it with Firefox. It is impossible to open the downloaded tgz afterwards. It is totally screwed up.
How about re-installing that Apache server from scratch?
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TWiki:Main.KennethLavrsen - 08 Mar 2007
having analysed what is in the file you download - the TWiki-4.1.2.tgz is in reallity a TWiki-4.1.2.tar.gz.zip. Ie the webserver zips the file when you download it. So someone has turned on compression on the webserver. We have had this problem before and had to turn it off. Now it seems someone has turned it on again.
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TWiki:Main.KennethLavrsen - 08 Mar 2007
There was an error in the configuration of the apache server on twiki.org: the directives AddType and AddEncoding have been
used incorrectly. Therefor the server didn't know anything about tgz and zip files, sending the wrong encoding to the browser. This error
was there for a long time and I am surprised that anybody was able to download a tgz at all before.
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TWiki:Main.MichaelDaum - 09 Mar 2007