MSYS2: Command works fine in terminal, but syntax error `(' in script

On Windows, with MSYS2, I want to add the file extension for an executable back to the output of which, if it has one. Here's a one-liner:

locAlt="$(which "mingw32-make")" ; echo "$locAlt" ; locAlt=("$locAlt"+(|.*)) ; echo "$locAlt"

When run in the MSYS2 MinGW terminal, this works just fine on my machine and prints:

/mingw64/bin/mingw32-make
/mingw64/bin/mingw32-make.exe

However, now I put the same line into a script (one-liner.sh):

#/bin/bash
locAlt="$(which "mingw32-make")" ; echo "$locAlt" ; locAlt=("$locAlt"+(|.*)) ; echo "$locAlt"

I tried both nano and Notepad++ (where you can choose line endings, and they're set to "UNIX (LF)", and checked the output of cat -v one-liner.sh, which shows the file as above (no ^M, as in this question ). However, when trying to run the file with ./one-liner.sh, I get the error

./one-liner.sh: line 2: syntax error near unexpected token `('
./one-liner.sh: line 2: `locAlt="$(which "mingw32-make")" ; echo "$locAlt" ; locAlt=("$locAlt"+(|.*)) ; echo "$locAlt"'

Why could that be?