Os mimetypes são determinados pelo que as man pages unix chamavam de 'magic numbers'. Em cada arquivo há um número mágico que determina o tipo de arquivo e o formato do arquivo. A extração abaixo é das man pages do comando de arquivos
The magic number tests are used to check for files with data in partic-
ular fixed formats. The canonical example of this is a binary exe-
cutable (compiled program) a.out file, whose format is defined in
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 executable, and which of several types thereof. The
concept of 'magic number' has been applied by extension to data files.
Any file with some invariant identifier at a small fixed offset into
the file can usually be described in this way. The information identi-
fying these files is read from the compiled magic file
/usr/share/file/magic.mgc , or /usr/share/file/magic if the compile
file does not exist. In addition file will look in $HOME/.magic.mgc ,
or $HOME/.magic for magic entries.
As páginas man do unix também mencionaram que se o arquivo não combina com um número mágico, o arquivo de texto é considerado ASCII / ISO-8859-x / não-ISO de 8 bits estendido ASCII (formato mais adequado)
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 Macin-
tosh and IBM PC systems), UTF-8-encoded Unicode, UTF-16-encoded Uni-
code, 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 iden-
tified as ''text'' because they will be mostly readable on nearly any
terminal
Sugestão
use o comando mimetype
em vez do comando de arquivo
mimetype temp.csv
link da web para escavação adicional
http://unixhelp.ed.ac.uk/CGI/man-cgi?file