Use like below in xml file.

as per Odoo11 

<field widget="binary" name="datas" filename="datas_fname"/>

                  <field name="datas_fname" readonly = "1" invisible="1"  force_save="1"/>

Get the uploaded file in py file.

curr_obj = self  

        if not curr_obj.datas:

            raise UserError(_('Please Choose The File!'))

        file_name = curr_obj.datas_fname

        print "curr_obj.datas_fname here",curr_obj.datas_fname

        print "file namr here",file_name

        fname = str(file_name.split('.')[1])

        if fname != 'xls':

            raise UserError(_('Please Choose The File With .xls extension and proper format!'))

        try:

            val=base64.decodestring(curr_obj.datas)

            fp = StringIO.StringIO()

            fp.write(val)     

            wb = xlrd.open_workbook(file_contents=fp.getvalue())

            wb.sheet_names()

            sheet_name=wb.sheet_names()

            sh = wb.sheet_by_index(0)

            sh = wb.sheet_by_name(sheet_name[0])

            n_rows = sh.nrows

Hi Vitaly!

The answer posted by Prakash should work fine, i would just change this part 

<group>
<field name="edi_filename"/>
<field name="edi_data" filename="edi_filename"/>
</group>

to  

<group>
<field name="edi_filename" invisible='1'/>
<field name="edi_data" filename="edi_filename"/>
</group>

Because filename already shows up following Download in your view and as this is "readonly" you do really need to display it two times.

But with the code you're showing above i am not sure if the previous answers reached your expectation.

I advise you to go to have a look on the ir.attachment model in the Base module of odoo. You will find all the answer you want to find there.

However, you will indeed need a field filename in order to have a name (plus an extension?) to your downloaded file. 

In Wizard using the below code

Python File

_columns = {
        'edi_data': fields.binary('File Stream', readonly=True),
        'edi_filename': fields.char('File Name', size=32, readonly=True),
        }



_defaults = {
   'edi_filename': 'Invoice.txt',
 }

XML File

<field name="arch" type="xml">
            <form string="Form">
                <group>
                    <field name="edi_filename"/>
                    <field name="edi_data" filename="edi_filename"/>
             </group>