so the receiver can save the file with the got number and we don't need a database?
The LevelDB database is mostly used in blockchains to store block related data like block height and block hash as keys and it stores the block data as values. The structure of the data is customizable and any blockchain systems could easily customize it according to their needs.
The main reason for implementing LevelDB database in blockchain is to provide fast and highly efficient way for the retrieval of the block related data. This makes it far easier to perform operations related to block data and for transactions. It also helps in data integrity and consistency of the data. The database is capable of doing consistent reads and writes.
To learn further about the LevelDB use in blockchain you can visit the following links.