Blender uses True Type and Postscript Type 1 fonts (if memory serves)

You can set the default font path in User Preferences

In User preferences i have set to Windows/Fonts folder.
Blender see only default fonts.
All font blender dont see is ttf format.

Got an example for us of a font file that doesn't work?
Please attach it.

1 - Folder - Windows/Fonts in Directory Opus on Windows 10

2 - Windows/Folders in Blender (after Shift+a - text)

Check the font-type (file extension) which Blender does not see.
Try turning off the file-type filter (at the top right).
Try uploading a file from another directory, not Windows/Fonts.

It may also be that the fonts you see in Windows Explorer (Fonts folder) are actually in a different folder! Check it.

When i select a different font directory (no windows/fonts) Blender see all the fonts, both TTF and OTF format. And loads them without a problem.
When i right click on a font (which the blender can not see in the windows/Fonts directory) and click install (or install for all users) font appears in windows/fonts directory but blender do not see him. But in different directory he sees without a problem.
All installed fonts are in Windows/Fonts directory.

Personally, if the blender sees all the fonts in a different directory, this is not a problem for me.

I think when you install the font, it is not real copied to the Windows/Fonts directory, it is only "linked".

Other programs see those fonts without problem.

After run this command:

Volume in drive C is System
Volume Serial Number is CCB6-9196
File Not Found

Do they show up (the missing fonts) if you simply do dir C:\Windows\Fonts\ ?

Yes they show up (default fonts). But without those missing by blender fonts. So You are right fonts are linked and not actually in that directory.

@Brecht Van Lommel (brecht) I'm guessing this is not something we plan to support for the 2.8 release?