In 2015, as Box was struggling on the public markets, reports came out showing that some firms had severely marked down the value of their Dropbox holdings.

Dropbox is in a brutally competitive market, going up against Apple and Google for consumer file storage (photos and videos), and taking on a host of companies from Microsoft and Amazon to Box on the enterprise side. That's forced the company to shutter some big initiatives in its effort to find a money-making business model.

Dropbox has confidentially filed for an IPO, CNBC reported on Thursday. In April, Bloomberg, which first broke the news about the confidential IPO filing, reported that Dropbox was profitable, excluding some taxes and other items.

But as the cloud file sharing and document collaboration company prepares to go public, investment firms have been gradually marking the price up They now value the company at between about $6.6 billion and $8.5 billion, according to a CNBC analysis of regulatory filings.

It's been four years since investors valued Dropbox at $10 billion . Since then, in the eyes of mutual funds and other shareholders, the company has been unable to justify that price.

The recent increases in Dropbox's price suggest that it might be a better time for the company to go public. But it will still need to grow profitably and excite the public markets to return money for many of its investors.

After going public, Dropbox could wind up with a market cap below or equal to its $10 billion private valuation from 2014, said Rohit Kulkarni, managing director for private investment research at SharesPost.

He estimates that the company generated about $1 billion in revenue last year, with 25 percent growth and close to two-thirds of sales coming from enterprises.

"The bigger part of the story they would probably be looking to tell is they do not need a capital influx, given their financial profile," Kulkarni said. "That would be the key argument that they would want to make to deserve a premium valuation, as opposed to many other cash-burning unicorns."

Here's a rundown of some investment firms that have changed how they value Dropbox:

BlackRock estimated that its shares in Dropbox were worth $18.23 each as of April 2015, but two and a half years later, at the end of October 2017, they were down 27 percent to $13.30. That's up from $10.87 a year earlier. BlackRock's global allocation fund got its stake in Dropbox in the company's January 2014 funding round.

Fidelity's Small Cap Growth K6 Fund has held stock in Dropbox since 2012. While at the end of July, the price was down 30 percent from early 2015, at $13.40, it was up from $11.10 a year earlier.

GSV Capital, which got in Dropbox's series A-1 round, prices its shares 45 percent below where they were in mid-2015. The stock was pegged at $12.58 at the end of September. But that's up from $10.05 per share at the end of 2016.

Hartford Funds' Capital Appreciation Fund, which invested in Dropbox in 2014, priced the shares at $19.84 in October 2015. One year later the price fell to $19.10, and most recently it was down to $16.20.

Principal's LargeCap Growth Fund I valued Dropbox shares at $19.10 in April 2015 after buying them in 2014, and they were down 33 percent to $12.79 at the end of October. That's up from $12.37 in April 2016.

USAA, whose Science & Technology Fund bought shares in Dropbox in 2012, valued them at $19.10 in April 2015. The price was marked down to $13.02 as of October, which was up from $11.03 a year earlier.