astextplain: restore passthrough for plaintext DOC/DOT files

The fix for git-for-windows/git#5641 broke
diffs for plaintext DOC and DOT files:

    $ echo Test > test.doc
    $ git add test.doc
    $ git diff --cached
    E: unsupported filetype test.doc

Since `file` correctly identifies the mimetype of such files as
`text/plain`, this can be fixed by simply covering this case and
printing the file content as it was done previously.

Signed-off-by: Claudio Hoffmann <[email protected]>

Author: ClaudioHoffmann

git-extra/astextplain

@@ -21,7 +21,7 @@ case "$(file --brief --mime-type "$1")" in
 		out=$(pdftotext -q -layout -enc UTF-8 "$1" -) && sed "s/(\^M$)|(^\^L)//" <<<$out || cat "$1"
 		;;
 	# TODO add rtf support
-	application/rtf | text/rtf)
+	application/rtf | text/rtf | text/plain)
 		cat "$1"
 		;;
 	*)