The type of files stored in that folder depend on the application - i.e. it's up to the app developer to store whatever they want there - it may be an Access file, an SQLite file, or maybe just an XML or JSON file. You can usually tell by the file extension what sort it is.
I've been using tinyMCE with the moxiemanager plugin for years, but their pricing structure now is such that I may as well fork out the $1K or so for Telerik or ComponentOne, or similar (and get the added benefit of their other controls as well.). If there's a free on even better, of course, but I haven't seen any that come with a good file manager. Wondering what recommendations people may have... posting this here because it must integrate with ASP.NET winforms webforms applications.
... a decent server file manager (such as moxiemanager) is essential. Moxie also includes some basic image-editing functions, which is useful. But, of course, ease of use of the HTML editor by users not proficient is HTML is key. One place tinyMCE falls down is with bullet lists, ordered or unordered - it's never been good at handling them, and even their latest incarnation isn't brilliant, so any improvement on that would be a plus. One thing tinyMCE is good at though (now, finally) is handling content pasted in directly from Word, and that is huge plus, users being who they are...
Anyway, be interested to hear of your experience and thoughts.