Not enough permissions to add attachment to Invoice

Hey, I have been trying to automate some attachment processing and hit an unexpected permissions issue. My goal is to automate the upload of some files as attachments of Invoices. My steps were the following:

Created a new self-client through the Developer Space UI
Downloaded the self-client
Got the Refresh token and first access token
Sent a POST request to: https://books.zoho.com/api/v3/invoices/{INVOICE_ID}/attachment

POST req; Upload attachment

POST /api/v3/invoices/INVOICE_ID/attachment?organization_id=ORG_ID HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate, br
Authorization: Bearer
{ACCESS_TOKEN}
Connection: keep-alive
Content-Length: 16045
Content-Type: multipart/form-data; boundary=697684a1c19f4bf4ada93be359205fb8
Host: books.zoho.com
User-Agent: HTTPie/3.2.1

+-----------------------------------------+
| NOTE: binary data not shown in terminal |
+-----------------------------------------+

With the following command:

http --form --verbose post https://books.zoho.com/api/v3/invoices/{INVOICE_ID}/attachment organization_id=={ORG_ID} Authorization:"Bearer {ACCESS_TOKEN}" attachment@test.png

And got:

{
"code": 104003,
"message": "You don't have permission to perform this operation. Please contact your Administrator."
}

This is the scope I'm using for the self-client:

"scope": [
"ZohoBooks.invoices.CREATE",
"ZohoBooks.invoices.READ",
"ZohoBooks.invoices.UPDATE",
"ZohoBooks.invoices.DELETE"
],

(I concatenate all those values in a single line, separated by comma).

Notes:

I can successfully send a READ (get) request to the same invoice_id
The same behavior is observed in other Objects (can READ, cannot WRITE)

I'm guessing a box needs to be checked somewhere throughout the Admin Panel?

Bonus question: According to the docs (here), one can only upload gif, png, jpeg, jpg, bmp and pdf files. Is there a reason for this? I would like to be able to upload xlsx files, as can be easily done through the UI.

Thank you :)

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