OS X In Depth: Documents where "Change Application" is disabled

There appear to be two main types of documents in Mac OS X.

One type can change its attributes by changing its suffix. A change in suffix will also change its icon. You can try this with documents created by TextEdit. These are also the documents in which the "Change Application" button is enabled in Show Info. In fact, changing the suffix can potentially change the application that will be launched when you double click the icon.

In the other type of document, changing a suffix has no effect. Indeed, a suffix not even needed for the file to be recognized by its creating application. In these cases, the Change Application button is disabled. It appears that documents created by Classic apps fall into this category (and likely involves the Type and Creator codes used by these applications).

We had a problem where a .dmg file was identified by the Finder as a SimpleText document. While we could force the file to launch Disk Copy, by selecting Disk Copy from the "A specific application" option in Show Info, the Change Application button was disabled, so we could not make a general change that would hopefully get the Finder to respect the .dmg suffix and change to the correct icon etc. We are investigating Mac OS X utilities that can help here (we want to see if we can fix it without going back to Mac OS 9).

A related oddity: We have both AppleWorks 6.1.2 and 6.0.4 on the same partition (6.0.4 is in the Mac OS 9 folder; 6.1.2 is in Mac OS X). When we save a document in AppleWorks 6.1, it saves it as a 6.0.4 document. Thus, if we then double-click the document, it opens 6.0.4. We have found no way to fix this.

See this MacFixIt Forums thread for some related material.