Minecraft will generate quite a few files in your world folder when you create a world. Most of these files are not needed and by deleting them you can reduce the file size when it comes to uploading your world.
Below is a table which displays all the files that may be generated in your world folder, along with a description and whether or not the file is required.
|YES||Stores global information about the world such as name and generation type|
|YES||Contains all the region files of your world|
|DEPENDS||Stores map data for craft-able maps -- remove if you are not using custom map items|
|NO||A backup of level.dat before the map was converted from the MCRegion world format to Anvil|
|NO||A backup level.dat|
|NO||A timestamp when the level was last accessed|
|or||NO||Contains files which store the individual states of each player|
|NO||Stores information about Villages in the world|
|NO||Contains all the region files for the Nether and The End|
|NO||Contains achievements and other stats|
|NO||Generated by MCEdit|
|NO||Generated by MCEdit/NBTEdit|
|NO||Generated by Not Enough Items|
|NO||Generated by Single Player Commands|