Change the 'enctype' in the form tag

[ShMiL]

I need to change the enctype in the form tag from the default to the "multipart/form-data" when someone click on button A.
I tried:
onClick="this.form.enctype='multipart/form-data';"

But it doesn't work, does anyone have a good idea for me?
thanks.

[glenngv]

onClick="this.form.encoding='multipart/form-data';"

[ShMiL]

Great!
Thanks alot!