The case of the zipped DMG
The preferred way for distributing OS X applications is through disk images, or DMG files. The great thing about DMG files is that they are self-contained. In most cases, you can even lauch the application from the mounted image itself.
Another good thing about disk images is that they support compression (and even encryption). Decompression happens on the fly as you open files on disk.
So why do we see zipped disk images files on the net ? It seems rather pointless if you enable compression on the disk image itself. You download a 1 Mb .dmg.zip file, unzip it to .dmg 1 Mb file and then mount it. It takes more time, doesn’t save bandwidth and requires additional cleanup by the user. It’s like zipping QuickTime movies or audio files.
I suspect the reason for this nonsense is the way file servers recognize DMG files. Because the dmg extension is pretty unknown outside of the mac community, there’s a chance the header sent by the server would be incorrect.
The result is garbled text on the browser instead of a download progress bar. ZIP is a recognized extension pretty much everywhere and thus is less likely to get incorrect headers.
If indeed it is a case of browser support, then I would argue that it more an issue of poor server administration. A standards-compliant browser should read it in plaintext is the MIME type is incorrect (or the browser does not have its own MIME definition for the format). However, it is up to server admins first and foremost to properly set and associate MIME types in their server configs. It seems to me this would eliminate a lot of issues.
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