Map type of doucments

If the vault has only a small amount of content, you can search by file type to get a rough estimate of the number of each type of file. You can do this via the Files tab under Additional Conditions. This doesn't work for larger vaults since M-Files will only show something like "10,000+" as the count if you reach the search limits.

With larger vaults you probably need to use the Reporting module to get all the data into an external reporting database and then run reports on that data. I don't recall off the top of my head if the file extensions are part of the standard metadata that you can export, though.