You may achieve your goal using rewriting create and write method where you can put validation for extension of the file. You can have a look below code as a reference:

import pathlib
from odoo.exceptions import ValidationError

def write(self, vals):
    ext = 0
    for attachment in vals['attachment_ids'][0][2]:
        file_name = self.env['ir.attachment'].browse(attachment).name
        if pathlib.Path(self.env['ir.attachment'].browse(attachment).name).suffix == '.pdf':
            ext += 1
    if ext != len(vals['attachment_ids'][0][2]):
        raise ValidationError('You can upload only pdf files!')
    super().write(vals)
    return True

Thanks & Regards,

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