Web application – What types of file icons should be used in web applications?

In short, the "unofficial" icons.

From my experience as a user, the "unofficial" approach is the most common (examples: Google Drive, OneDrive). In addition to this, there are a number of problems with the "official" approach, which is solved using the "unofficial" approach:

  • Lack of internal consistency, as you mention
  • Copyright problem
  • Have to keep up with the ever-changing icons apps
  • And what to do with generic files, such as JPEG, AVI, TXT?

The big advantage of the "official" approach is that the user can recognize a file type faster, but I do not think it overrides the issues.