[feature-request] Proper handling of RTFD documents in Mac PKG format

[foreignmeloman]

• Electron-Builder Version: 23.0.3

• Node Version: 16.x

• Electron Version: 18.2.3
• Electron Type (current, beta, nightly): current

• Target: mac

Since adding pictures via <img src="data:image/png;base64,..."> tag does not work for Mac PKG format when specifying a html file in .pkg.welcome, .pkg.license or .pkg.conclusion options, it would be really nice if electron-builder supported RTFD documents too.

Currently when I try to specify an RTFD file (well, it's actually a directory) in .pkg.conclusion, the builder throws this error:

  ⨯ Exit code: 1. Command failed: productbuild --sign XXXXXXXXXXXXXXX --distribution /path/to/distribution.xml /path/to/app.pkg
productbuild: error: Cannot write product to "/path/to/app.pkg". (The file conclusion.rtfd couldn’t be opened.)

There is a possible workaround by using process described in this StackOverflow answer with .mac.afterSign option, but I think that we need a proper solution here.

#4432 is also a related to this, but for some reason was closed by the author.