Firstly its important to understand that assembly files are all linked to the part files. These files do not hold all the information of the individual parts. When opened an assembly file searches for the part files linked to it and draws information from it. So in case you have uploaded an assembly file alone, the assembly doesn't get the info it needs and hence doesn't work!
The process is quite simple, but the aim of this tutorial is to make people realize how important it is to avoid making this error.
I'm going to show you how to do this using my model Shock Absorber. This has 3 part files and an assembly. The file named 'ShockAbsorber' is the assembly.
Select all 4 files. Right click and select Add to zip file.
A new zip file which contains all the 4 files is created.
While uploading the zip file alone needs to be added.
This file can be used by all and your ingenious model will get all the credit it deserves!