It is a local file produced by an app or a previous sync conflict. To give you a precise answer, could you tell me:
Dropbox stores configuration data, including file/directory listings and hashes, in SQLite databases. It's possible (51) refers to a specific version or node ID in a database analysis.
When Dropbox synchronizes files, if multiple versions exist, it creates a "conflict" copy, often renaming files to filename (number).extension .
Red sguigglies in TypeScript file for all third party libraries
In TypeScript (a programming language), .d.ts files provide type information about a JavaScript library. If you see Dropbox.d.ts or similar, it's defining how to interact with the Dropbox API.