What does “magic tests” mean for the file command

I can't think of an all-in-one tool, but there are programs that can cope with a large array of files of a given category.

For example, p7zip recognizes a large number of archive formats, so if you suspect that a file is an archive, try running 7z l on it.

$ 7z l ta12b563enu.exe
…
Type = Cab
Method = MSZip
…

If you suspect that a file is an image, try ImageMagick.

$ identify keyboard.jpg.gz
keyboard.jpg.gz=>/tmp/magick-XXV8aR5R JPEG 639x426 639x426+0+0 8-bit DirectClass 37.5KB 0.000u 0:00.000

For audio or video files, try mplayer -identify -frames 0.

If you find a file that file can't identify, you might make a feature request to the author of your magic library.

People had been writing scripts (and possibly C programs) to run file on a file, capturing the output with $(file foobar) or popen(), and doing a string match check to see whether the output from file contained (or ended with) the word “text”.  Then the developers of the Berkeley Software Distribution (at the University of California, Berkeley) did as described and caused all those scripts and programs not to recognize shell script files as text files.