Hi @maximvanneste,
Could you please share a bit more about your scenario?
Do you want to add a new record with a attachment into your SP list?
Further, do you want to upload attachments into SP list item using code?
If you want to upload attachments into SP list item using code directly, I afraid that there is no way to achieve your needs in PowerApps currently.
If you would like this feature to be added in PowerApps, please submit an idea to PowerApps Ideas Forum:
https://powerusers.microsoft.com/t5/PowerApps-Ideas/idb-p/PowerAppsIdeas
I agree with @RusselThomas's thought almost. Currently, if you want to upload attachments to SP list item, I think the Attachments control could achieve your needs, but the upload functionality of this control only works within a form (Please check here).
More details about the Attachments control, please check the following article:
Attachments control
In addition, you could also take a try to create a flow (which triggers from an app) to upload an attachment to SP list item. More details about triggering a flow from an app, please check the following article:
Start a flow from an app
Please check and see if the following blog would help in your scenario:
https://www.techmikael.com/2017/05/an-even-more-clever-workaround-for.html#comment-form
Best regards,
Kris