Why does php file_info return inode/x-empty for empty text files?

Question

I am currently working on a class that wraps the finfo_file function.

The following script returns inode/x-empty for all empty text files:

$finfo = finfo_open(FILEINFO_MIME_TYPE);
echo finfo_file($finfo,'/path/to/text_file.txt');

Tested in the following environments and received the same results.

WAMP
Windows 7
PHP 5.4.27
APACHE 2.2.22

Linux
Ubuntu
PHP 5.5.10
APACHE 2.4.9

My goal is to setup the class so that I can white list file types. If a MIME type of text/plain is white listed, the empty text file would fail since it returns a MIME type of inode/x-empty.

Is this default behavior for the finfo_file function?

Answer 1

Yes, because the file extension doesn't define the file type. You can, in example, change the extension of a video file from .mp4 to .txt, and still play the video in a player. Windows does handle file extensions a bit more strictly, but in unix systems the extension is more like a type-hint for users then that it really means something for the system itself (exceptions are there though).

The file info functions look to the contents of a file and try to determine the mime-type from what it finds there.

If you want to white-list text/plain, but also empty text files, you could do something like this, using pathinfo():

if($mime_type == 'inode/x-empty' && pathinfo($file_name, PATHINFO_EXTENSION) == 'txt') {
    // whitelist
}

Answer 2

From the documentation:

Returns a textual description of the contents of the filename argument, or FALSE if an error occurred.

(emphasis mine). As you can see, it's based on the contents, not the name.