Where is the location of the mime plugin files in mac and linux?

Question

I was just curious as to how mime types were read/interpreted by local applications including the browser. Are the plugins for reading the mime type build into each application, or is there a special system folder in the OS that the application refers to when interpreting a mime type?

RFC uses the character-charts as references when defining what a MIME type is:

(1) textual message bodies in character sets other than US-ASCII

Although MDN makes it sound like it uses the content-type you would find in things like HTML

Does something like content-type=image/jpeg or content-type=application/javascript use the UTF-8 char chart to determine their charsets (The glyph) while something else does the logic to determine what those char glyphs should be interpreted into?

OR does this mean that each content-type has their own special charchart (like utf-8 -> js-8????) that does both glyph conversion of character and logic interpretation of char glyph into binary?

Why does it sound like charcharts and content-type both mean MIME? Where is the folder path for both Mac and Linux systems containing content-type charts / interpretation logic?

Answer 1

On macOS you can use file --mime "/path/to/filename" to report the mime type of a file.

The man page for file (see here) sheds some light on what happens, under-the-hood, before the mime type lookup:

 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''.