在 macOS 上，您可以使用
file --mime "/path/to/filename" 命令来报告文件的 MIME 类型。
file 的 man 页面（请参见
此处）阐明了在进行 MIME 类型查找之前，在幕后发生了什么。
file tests each argument in an attempt to classify it. There are three
sets of tests, performed in this order: filesystem tests, magic tests,
and language tests. The first test that succeeds causes the file type to
be printed.
The filesystem tests are based on examining the return from a stat(2)
system call. The program checks to see if the file is empty, or if it's
some sort of special file. Any known file types appropriate to the sys-
tem you are running on (sockets, symbolic links, or named pipes (FIFOs)
on those systems that implement them) are intuited if they are defined in
the system header file <sys/stat.h>.
The magic tests are used to check for files with data in particular fixed
formats. The canonical example of this is a binary executable (compiled
program) a.out file, whose format is defined in <elf.h>, <a.out.h> and
possibly <exec.h> in the standard include directory. These files have a
``magic number'' stored in a particular place near the beginning of the
file that tells the UNIX operating system that the file is a binary exe-
cutable, and which of several types thereof. The concept of a ``magic''
has been applied by extension to data files. Any file with some invari-
ant identifier at a small fixed offset into the file can usually be
described in this way. The information identifying these files is read
from the compiled magic file /usr/share/file/magic.mgc, or the files in
the directory /usr/share/file/magic if the compiled file does not exist.
If a file does not match any of the entries in the magic file, it is
examined to see if it seems to be a text file. ASCII, ISO-8859-x, non-
ISO 8-bit extended-ASCII character sets (such as those used on Macintosh
and IBM PC systems), UTF-8-encoded Unicode, UTF-16-encoded Unicode, and
EBCDIC character sets can be distinguished by the different ranges and
sequences of bytes that constitute printable text in each set. If a file
passes any of these tests, its character set is reported. ASCII,
ISO-8859-x, UTF-8, and extended-ASCII files are identified as ``text''
because they will be mostly readable on nearly any terminal; UTF-16 and
EBCDIC are only ``character data'' because, while they contain text, it
is text that will require translation before it can be read. In addi-
tion, file will attempt to determine other characteristics of text-type
files. If the lines of a file are terminated by CR, CRLF, or NEL,
instead of the Unix-standard LF, this will be reported. Files that con-
tain embedded escape sequences or overstriking will also be identified.
Once file has determined the character set used in a text-type file, it
will attempt to determine in what language the file is written. The lan-
guage tests look for particular strings (cf. <names.h>) that can appear
anywhere in the first few blocks of a file. For example, the keyword .br
indicates that the file is most likely a troff(1) input file, just as the
keyword struct indicates a C program. These tests are less reliable than
the previous two groups, so they are performed last. The language test
routines also test for some miscellany (such as tar(1) archives).
Any file that cannot be identified as having been written in any of the
character sets listed above is simply said to be ``data''.
