File storage and sharing company Dropbox has filed confidentially for an IPO, according to Bloomberg. Goldman Sachs and J.P. Morgan are leading the offering.

Why it matters: Dropbox is one of the most-valuable tech companies to remain privately-held, having last raised money at a $10 billion valuation.

The big question is how Dropbox will be valued by the public markets, if and when it goes public (confidential filings don't start any sort of clock, and often suggest the pricing could be well in the future). Since that last raise, some of Dropbox's mutual fund investors have marked it down substantially, although rival Box has seen its share price more than double in the past year.

San Francisco-based Dropbox has previously said that it is cash-flow positive with more than $1 billion in annual sales. For context, Box has just south of $500 million in annual revenue and a market cap of $2.88 billion.