File type to application association and plug ins.

Question

Hello,
I'd like to find more technical information on the following related topics:
- First of all, how are file types identified: File extension? Unix magic numbers? Both?
- File type to application association information: Where is it stored? pfile? where?
- Link to file type to "quicklook" plug in (how does quicklook figure out which plugin to use?) Where is it stored? Is it dynamic (finder scans its QuickLook folder and builds on the fly associations?)
- Link to "preview" icon generation: how does the finder know how to generate a preview icon, and with which application/plugin depending on the file type.
I don't need "user guide" type of thing. I'd like to know for instance where "Open with..." menu item gets its list of application that can open a certain file type.
Is there a reference guide/document/tech note where Almighty Apple explains this.
Thank you very much for your input.

Answer 1

I've only done a little bit of poking around in QuickLook. I do know that the QuickLook generators, like applications, have an info.plist file in the bundle that declares the file types that are handled, but I doubt they actually do the heavy lifting of drawing the preview: they seem just too small to do it. Thus, the actual executable for the PDF generator is a mere 52kb. Whether the info.plist is polled, and if so when, and whether the information is then stored in a cache or database somewhere I don't know. It would seem logical that this is the case.
As for what is actually drawing the previews, I would guess that is done by QuickTime. The Finder has been able to present icon previews of many things since the first introduction of OS X. Indeed, before the advent of QuickLook the Finder was able to draw previews of jpegs, psd files, gifs, and so on. AFAIK, it could draw a preview for anything that QuickTime could open. My guess, and this is JUST a guess, is that QuickLook may indeed now also be involved, as well as QuickTime, and that the integration of three things--Finder, QuickTime and QuickLook--does not work as well as one would hope. I say this because Finder windows now render their contents rather more slowly than was the case prior to the introduction of QuickLook. Many people have commented on this, and I have noticed it too.
As to whether icons are "cached"--I've never reached a conclusion about this. I do know that pre-Leopard the Finder would display the contents of a window in icon view dang near instantaneously IF the files had their own custom thumbnails, but would take a noticeable amount of time to display things if they did not, so that it had to render the thumbs itself. It also seemed like once a window had been opened it rendered more quickly on subsequent openings. Maybe. It no longer renders ANYTHING instantaneously--if I open a folder with all Photoshop files, all having their own custom thumbs, it hesitates for a second or so before presenting the icons. This did not use to be the case. Opening the same folder again one does get the old behavior of instant icons. That sounds like a temporary cache is being created somewhere. I say temporary, because if you relaunch the Finder, then open the folder again, you once more have the one second pause before the icons appear.
If there is a cache it is not the .DS_Store file. That file does store information about window properties, such as which view is chosen for the window, what the size and position is, what options are checked for the display of the window, and, oddly enough, the Spotlight Comments for files that are present. The file just isn't big enough to be storing thumbs--the .DS_Store file for a folder having 61 jpegs, without custom thumbs, set to icon view is a mere 42kbs, and one for another folder with 81 jpegs without custom thumbnails set to icon view is a tiny 24kbs.
Francine
Francine
Schwieder