Field type "file"

Hello,
My question is a bit out of context, but I will need your opinion.

I am developing a "Market Place" extension for my EasySocial site.
It works fine, however, I can not limit the sending of image to JPG format..
I try to use: accept = "image: jpg",
in:
<field name = "photoprincipal" type = "file" label = "COM_ESOFFERS_FORM_LBL_ANNONCE_PHOTOPRINCIPALE" description = "COM_ESOFFERS_FORM_DESC_ANNONCE_PHOTOPRINCIPALE" hint = "COM_ESOFFERS_FORM_LBL_ANNONCE_PHOTOPRINCIPALE" accept = "image / jpeg" >

Would you have an idea for limited sending it to a jpg format?

Best regards.

Eric