I've found a solution with no JQuery, only EXtJS... It seems to work I have to test deeper but for those who are interested :
xtype: 'textfield',
inputType: 'file',
name: 'files[]',
fieldLabel: 'select files to upload ',
autoCreate: { tag: 'input', type: 'text', size: '70', autocomplete: 'off', multiple: 'multiple' },
The trick is in the autoCreate that allow to add the "multiple: 'multiple' " piece of tag, and not forgot to name the field as a table (files[])... This way allows to multi select files using the traditional way.... (easy for end-users)... Of course you have to adapt your php code to deal with the array of arrays (files[]) but it's out of the scope for this thread ^_^
Cheers