Cannot upload pdf using the Insert image button

Can I use the Insert image button to upload others mimes than jpg/png/gif?
Following advice in https://anspress.io/resources/faq/how-to-allow-custom-file-upload-in-anspress/ I tried to:

• Add new mimes in functions.php
• Add new mimes in dedicated plugin (priority issue)
• Insert new mimes in /includes/upload.php

None of them worked. I still get the ‘File type is not allowed to upload’ message.
Do I have to add a special custom field for other mimes than jpg/png/gif?