Can't set the Content-Type

As Katalon.ini should speciify -Dfile.encoding=utf-8 explains, you should try editing C:\Users\myname\Katalon_Studio_Windows_64-x.x.x\katalon.ini so that it includes a line of

-Dfile.encoding=Shift-JIS

You need to stop and restart KS.

Please note, if you change the katalon.ini file this way, it is likely that you would encounter character-garbling when you work on other URLs which have charset=utf-8.

If you want to set `file.encoding=Shift-JIS` just temporarily for a particular URL, then you may try setting Java SystemProperty `file.encoding` dynamically by
def previousCharset = System.getProperty('file.encoding')  // save the previous charset (possibily UTF-8)
System.setProperty('file.encoding', 'Shift-JIS')
... do anything for your target URL
System.setProperty('file.encoding', previousCharset)   // restore the previous charset 

I do not see your test case scripts, so that I have no idea where/how you can do this.