Bash script: check if a file is a text file

1. From the fact that it says file $attachment rather than file "$attachment", I guess your script cannot handle filenames that contain spaces.  But, be advised that filenames can contain spaces, and well-written scripts can handle them.  Note, then:

   $ file "foo bar"
   foo bar:  ASCII text
   
   $ file "foo bar" | cut -d' ' -f2
   bar:
   

   One popular and highly recommended approach is to null-terminate the filenames:

   $ file -0 "foo bar" | cut -d $'\0' -f2
   :  ASCII text
   

2. The file command makes educated guesses about what type of file a file is.  Guesses, naturally, are sometime wrong.  For example, file will sometimes look at an ordinary text file and guess that it is a shell script, C program, or something else.  So you don't want to check whether the output from file is ASCII text, you want to see whether it says that the file is a text file.  If you look at the man page for file, you will see that it more-or-less promises to include the word text in its output if the file is a text file, but this might be in a context like shell commands text.  So, it may be better to check whether the output from file contains the word text:

   isFile=$(file -0 "$attachment" | cut -d $'\0' -f2)
   case "$isFile" in
      (*text*)
         echo "$attachment is a text file"
         ;;
      (*)
         echo "$attachment is not a text file, please use a different file"
         ;;
   esac
   

The problem occurs in cut -d\ -f2. Change it to cut -d\ -f2.

To cut, the arguments look like this:

# bash: args(){ for i; do printf '%q \\\n' "$i"; done; }
# args cut -d\ -f2
cut \
-d\ -f2 \

And here is the problem. \ escaped the space to a space literal instead of a delimiter between arguments in your shell, and you didn't add an extra space so the whole -d\ -f2 part appears as one argument. You should add one extra space so -d\ and -f2 appear as two arguments.

To avoid confusion, many people use quotes like -d' ' instead.

P.S.: Instead of using file and making everything ASCII, I'd rather use

if file "$attachment2" | grep -q text$; then
    # is text
else
    # file doesn't think it's text
fi

case $(file -b --mime-type - < "$attachment") in
  (text/*)
     printf '%s\n' "$attachment is probably text according to file"
     case $(file -b --mime-encoding - < "$attachment") in
       (us-ascii) echo "and probably in ASCII encoding"
     esac
esac