fix hidden webp thumbnail throwing MIME type error in downloader

[Araxeus]

Fix #315

The Problem

youtube sometimes hide the real picture filetype, for example in this link:

https://i.ytimg.com/vi/tmntIJSf0Dw/hqdefault.jpg?sqp=-oaymwEcCNACELwBSFXyq4qpAw4IARUAAIhCGAFwAcABBg==&rs=AOn4CLC5b9hkS4_861N7ooUq4fUGX7RLCw

the .jpg is a lie, this file is a webp - and Electron's nativeImage doesn't support this type.
(when trying to save the picture in a browser, it will auto suggest name hqdefault.jpg.webp..)

this results in an empty native image - and setting APIC frame to an empty buffer:

youtube-music/plugins/downloader/back.js

Lines 66 to 68 in 3485d26

	writer.setFrame("APIC", {
	type: 3,
	data: coverBuffer,

results in a thrown error: Unknown picture MIME type

The Fix

fixed by removing everything that comes after the ".jpg" (only if needed - the nativeImage is empty)
in our example the result is:
https://i.ytimg.com/vi/tmntIJSf0Dw/hqdefault.jpg
which is a valid jpg file

Also check before creating coverBuffer that nativeImage isn't empty to avoid potential error throwing

[th-ch]

Looks good, thanks for the fix!