fix, feat: Add MIME type to HTTP file transfer API

[Mishura4]

As I found out the hard way on my bot B-12 while adding a command to add stickers, it turns out the sticker create API expects the file it receives to have a MIME type that corresponds to its data. I spent hours trying to troubleshoot my bot's code, but while digging into DPP code it became clear that the application/octet-stream MIME type it sent by default was what was causing the issue, as changing it to image/png allowed it to be accepted by the API.

This pull request adds MIME types to APIs that deal with raw file uploading through the REST API. This includes data members in dpp::http_request, in dpp::message, and function signature changes. MIME type defaults to application/octet-stream if unspecified by the user.
This effectively :

• Fixes the creation of new stickers through the REST API
• Allows attachments to have their MIME type set, which allows them to be displayed in corresponding ways in the Discord app. For example a file with text/plain as type will have its text shown in the app, a file with application/octet-stream will be available for download, and so on.

Changes of note :

• New defaulted parameter to dpp::http_request::post_rest
• New defaulted parameter to dpp::message::add_file
• New defaulted parameter to dpp::http_request's constructors
• New data fields in dpp::message and dpp::http_request

⚠️ This breaks ABI, as function signatures were changed around and data members were added. API is preserved through the new parameters being defaulted.

Tested on my own bot, on MSVC, clang-cl, Linux-GCC. Unit tests also pass (I also added the new MIME type parameters to it).

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[Commandserver]

What sticker API do you mean? This one?: https://discord.com/developers/docs/resources/sticker#create-guild-sticker

[Mishura4]

This one yes, and subsequently dpp::cluster::guild_sticker_create.

[Commandserver]

What was your code that didn't work? And what was the error message?

[Mishura4]

Starting here, guild_sticker_create gets called by sticker_add below, defined here (the executor is basically a wrapper for a mutex/condvar and std::invoke).

The error message was simply "Invalid Asset".

[Mishura4]

I tested extensively, the content I retrieved through the sticker URL was valid, the resized image as well, I kept writing my data to a PNG file and it was working fine. Even uploading those processed PNG files through the discord app worked, while my bot didn't, even after re-reading the PNG files and uploading that data. Changing the MIME type is the only thing that worked.

Discord was even accepting my files as an attachment to messages, which were displaying fine as well. It was only for stickers that it rejected it.

[Commandserver]

I'm just curious because i can't find any mime type in discord's documentation

[Mishura4]

Yeah 😆 I was really confused as well, my data was working fine, I thought I had all the requirements the API docs specified, except it kept rejecting my sticker through the bot. This could be a suggestion to send them for sure.