function extractContent(fileName, callback, extractFromMainContext) {
    // Ask the main thread context to provide the updated file content
    // We can't yet use node io to read, to utilize shells encoding detection
    extractFromMainContext.apply(null, [fileName]);
}
Extracts file content for the given file name(1st param) and invokes the callback handle(2nd param) with 
extracted file content. Content can be extracted locally from the file system used fs.readFile()
or conditionally from main context(brackets main thread) by using the 3rd param 

@param {String} fileName - fullpath of the document
@param {Function} callback - callback handle to post the content back
@param {Object} extractFromMainContext - content request handle wrapper from main thread