"Edit" always open Notepad, ignoring path på external editor.

It doesn't matter if I select "Internal editor" or if I enter "C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\devenv.exe" in Default file viewer and Default file editor - Notepad always launches for certain file types.

If I edit a CSS file Visual Studio is launched (yay) but if I edit an ASPX file Notepad is always launched (not intended).

Can this be fixed?

The default file editor is only used in cases where a file doesn't have a per-configured editor either in Windows or in FlashFXP.

You need to create a custom file association
From the main menu, Options > File Associations

For example you might add *.css to the File Mask and then select internal editor or any 3rd party text editor. Check the box to use this editor for file editing and adjust the upload action as needed and then click the Save button.