Review for Auto Zip Attachments by AlanUS
Rated 5 out of 5 stars
+1 to get the officially supported version for TB3 of this absolutely needed add-on
In the meantime, I have - succesfully so far - implemented the Max Version and AddUrlAttachment changes described earlier in the comments.
It requires to understand how to manipuate .jar files, which can be opened via any zip tool.
To create your own collections, you must have a Mozilla Add-ons account.