Steps to reproduce:

• Add {{#setmainimage:Filename.ext}} to a page and save it.
• Open the page in source mode to browser it's HTML

Actual result:

• no og:image tag in the HTML.

Expected result:

• og:image tag should be present in the HTML with the set image.

The reason the file namespace it not included in the parser function call is because the parser calls returns the name of the file, which then allows the call to be chained into a file call such: [[File:{{#setmainimage:Filename.ext}}|thumb|right|...]].