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 :)
--
Bartosz Dziewoński