This is a technical term, used in discussing computer data storage. Both forms, filepath and file path, are used, but which one is used is often dependent on context. While I can’t find any specific reference for usage in context, my experience has been that filepath, as an unhyphenated compound word, is generally used when discussing it as an entity (e.g., “You’ll need to set the filepath before writing out any data.”), but file path, as two words, is generally used when referring to a particular attribute of a file (e.g., “The file path of the JonesCo proposal is C:\Users\jsmith123\Documents\Proposals\JonesCo.docx.”
The general trend is how FumbleFingers described.
If in doubt, I type (or copy & paste) an expression as a single word into dictionary.com. It may tell me it knows that word. It may return it with a hyphen. If it doesn't recognise it, I then write it as two words.
That's the only way I know to be consistent in always writing words the same way.