Why is the option "Include in VSIX option" missing

I have a "Visual Studio Extension" project which I created using Visual Studio 2019.

If I add files to the project - for example a help file - I expect to see the option "Include in VSIX" in the properties window for that file. The option is missing in this project.

I have just created a new "Visual Studio Extension" project and added an additional file. In this case, the option "Include in VSIX" is present, so it looks like a problem in my specific project.

I guess there is something different in the project file. Does anybody know what it might be?

By the way, the project type GUID {82b43b9b-a64c-4715-b499-d71e9ca2bd60} which indicates an extensibility project is present.

In addition, if I patch the option <IncludeInVSIX>true</IncludeInVSIX> into the project file for the required file, the file does get included in the VSIX installation.

Solution

In the project file (.csproj) there is a list of project type guids. The list was:

<ProjectTypeGuids>
  {60dc8134-eba5-43b8-bcc9-bb4bc16c2548};
  {82b43b9b-a64c-4715-b499-d71e9ca2bd60};
  {FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}
</ProjectTypeGuids>

(actually all on one line).

These GUIDS mean (in the above order)

WPF project
Extensibility project
C# project

I have changed the sequence to

<ProjectTypeGuids>
  {82b43b9b-a64c-4715-b499-d71e9ca2bd60};
  {60dc8134-eba5-43b8-bcc9-bb4bc16c2548};
  {FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}
</ProjectTypeGuids>

(again, it is actually on one line)

That is, I have moved the GUID which indicates an extensibility project to the start of the list. Now the "Include in VSIX" appears.