Can't send a binary attachment to Slack using LogicApp

Question

I'm trying to use a logicapp to get the content of an email and post it to slack. By content I mean:

• the body of the email and other elements like From:, Subject:
• any attachment in the email (which usually are binary like PDF, Excel, image)
• the email itself saved in a blob as .eml file

Slack chat.postMessage API works without any problem to send any text element. This API has some attachment argument but doesn't seemto be designed or binary files (or not for files at all, only strings)

I've tried slack files.upload one but couldn't figure out the syntax, especially the syntax using a regular HTTP POST. Could find examples online using curl, Python, JS and C# SDK but I don't know how to translate them to HTTP POST just like I do with chat.PostMessage

I've tried the API on SOAP UI, using file as argument as per the documentation, and I've used it in different sections: in the header, in the body, and using the Attachment Tab, none of the work and always the same error message : no_file_data

Unfortuatelly slack documentation lacks of details. Here's what it says about files.upload:

You must provide either a file or content parameter. The content of the file can either be posted using an enctype of multipart/form-data (with the file parameter named file), in the usual way that files are uploaded via the browser, or the content of the file can be sent as a POST var called content. The latter should be used for creating a "file" from a long message/paste and forces "editable" mode.

In both cases, the type of data in the file will be intuited from the filename and the magic bytes in the file, for supported formats.

I could use alternatives like just saving the attachments in blobs and use Azure functions to send the file, but I want to understand what's the limitations before changing the method.

Any clue?