Hello, I have a little supplement after reading the first answer. To be on the safe side, the module will receive a different filename, and we should use the .endwith function to determine the end of the file. 

As shown in the following code:



In model:



file = fields.Binary(string='file', attachment=True)

filename = fields.Char()



@api.constrains('file', 'filename')

def get_data(self):

    if not self.filename.endswith('.pdf'):     # check if file pdf

        raise ValidationError('your error message')

    else:

        pass





In views:



<field name="file" filename="filename"/>



<field name="filename" invisible="1"/>

Unfortunately, there is no built-in option to add accept param by binary field definition so you have to do customization for it.
​
Good news is that in master new PR landed in the master to achieve it and available in next versions 14.0 :)

ref: https://github.com/odoo/odoo/pull/38351​    

Note: it's a client-side option so user still can upload any file by removing the filter from browse window. if you want strict restriction then detect file type from binary content at server side

Use the constrains decorator to examine the type of data being loaded. See constrains documentation

Example:

@api.constrains('name', 'description')
def _check_description(self):
    if self.name == self.description:
        raise ValidationError("Fields name and description must be different")