mime_content_type() reads PNG images as "text/plain"

[ppowell]

 echo mime_content_type("/images/myimage.png");

produces

text/plain

This only occurs whenever PHP is installed with -with-magic_mime enabled. On another system I maintain where this is not the case (I tested with function_exists('mime_content_type') and got FALSE of course), I default to my home-grown "mime_content_type" function that reads from the Unix file utility, and reads correctly. Is this an obvious bug or what did I do wrong this time?

Thanx
Phil