Fixing a file's "kind" on Mac OS X

Yesterday I had an annoying problem where I was emailed a file and then had to open it using a closed-source program on Mac OS X.  The issue was that the program only wanted to open a file of a certain "kind"; even though all the data was in the file and the file was in-tact the program would refuse to open it. 

Searching on google, there are several utilities that one can install to fix this metadata problem.  However, I was not the administrator of this computer as it was in a lab setting.  Here was a nifty solution.

I had already created several files from this program (Groups and Graphs).  Using a hexeditor I verified that the 'kind' information is definitely stored in the filesystem, and was not based on the contents of the files at all.  If I couldn't change the metadata, I figured I could copy the contents (but not the entire file object) of the file into a file that did have correct metadata.  Here's how this worked.  I had a file, old-good-file, of the correct type that opened.  My new file that wouldn't open was called new-bad-file.

Then in a shell I ran,

cp old-good-file new-good-file
dd if=new-bad-file of=new-good-file

 This way the contents of new-bad-file were moved to new-good-file without changing the metadata of new-good-file.  new-good-file inherited its metadata from old-good-file.  Now new-good-file works perfectly!