I've put together a modpack for 1.12.2 based around Minecolonies and MrCrayfish Furniture mod. I believe I have done everything correctly and the solutions I found online didn't work for me:
So I have Minecraft Forge version 1.12.2 - 22.214.171.12454 (universal) and I renamed it "modpack". Since it's a java file the .jar is an assumed extention, when I upload to drop dox "modpack.jar" displays with ".jar.jar" so I eliminated this with just "modpack" which now displays as modpack.jar.
"modpack".jar is in the bin folder, my compatible mods are in the mods folder, and the config files in the config folder.
All three of these folders are at the top level of the .zip meaning I highlight the folders themselves rather than a folder containing them.
The modpack is downloaded just fine so my link is not the issue.
For whatever reason I can't "insert image from url" so here is the imgur link with a pic of my modpack's zip file: https://imgur.com/a/d8B8Ylx