Continuing from last week's Tip about Open With, this week we'll look a little closer at the contents of the Open With panel, and especially at those last two applications, DiskProbe and FileTypes, that seem to always show up no matter what file you're opening:
First of all, let's look at the Relation column in the Open With panel. This is the panel presented on my system when I used Open With to open an HTML file named "index.html". The Relation column indicates that NetPositive is the preferred application for this file (which has a MIME type of "text/html", so that's what we'd expect).
Next it indicates that StyledEdit, Pe, and BeIDE all handle text files of any kind. This is because these applications advertise that they can handle the MIME supertype "text" (you can read more about MIME supertypes in a previous Tip, More About MIME). Open With will offer as a possibility any application which can handle the supertype of the file being opened.
Finally, we come to two applications, DiskProbe and FileTypes, which apparently are ready for anything. They can handle any file, meaning they can open a file no matter what its MIME type is. For this reason, they are called "superhandlers." (Any application which handles the "application/octet-stream" MIME type is a superhandler, because that MIME type represents any file.)
So what do these superhandlers do, anyway? DiskProbe is a file editor. It lets you edit the raw bytes of a file, directly on the disk. Be careful! There's no Revert menu choice in DiskProbe, and Undo will only affect your last change. If you start fiddling and get in over your head, there is no life preserver (though you can quit without writing your changes to disk). For that reason, while the non-technical user may occasionally find uses for it, it is definitely one of those tools best wielded by an expert.
FileTypes, when used to open an individual file, allows you to edit the MIME information about that file. You can alter the MIME type for the file, for example, changing it from "text/plain" to "text/html". You can also set the preferred application for this file, that is, change the application that opens the file when you double-click it:
Note that this is different from the Open and Make Preferred button in the Open With panel, in that instead of changing the preferred handler for all files with that MIME type, as the Open With panel does, using FileTypes on an individual file just changes the preferred handler for that single file. Very handy when you want to leave your system settings alone, but have a file that is an exception to the rule.
Next week: A Tip about BeOS Release 3.1!
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