You must have "ResourceFileModulePaths" set in your extension.json too, to make MediaWiki look for the file 'modules/ext.Iframe.js' in the right path. You don't mention whether you have it set or not. Like this:
"ResourceFileModulePaths": { "localBasePath": "", "remoteExtPath": "IFrame" },
(Where "IFrame" is the name of your extension's directory.)
Make sure to avoid typos, "Iframe" and "IFrame" are not the same, you used both in your example code and I don't know which is right :)