If you're on Linux, try file -i filename.txt
.
$ file -i vol34.tex
vol34.tex: text/x-tex; charset=us-ascii
For reference, here is my environment:
$ which file
/usr/bin/file
$ file --version
file-5.09
magic file from /etc/magic:/usr/share/misc/magic
Some file
versions (e.g. file-5.04 on OS X/macOS) have slightly different command-line switches:
$ file -I vol34.tex
vol34.tex: text/x-tex; charset=us-ascii
$ file --mime vol34.tex
vol34.tex: text/x-tex; charset=us-ascii
Also, have a look here.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…