WordJS doesn't return MIME type for images. I need to detect that for inserting images into the HTML canvas.Here are some links and code I started to work on when researching how to do this.

DetectMIMEtype.js

    // http://stackoverflow.com/questions/16245767/creating-a-blob-from-a-base64-string-in-javascript, thank you Jeremy Banks
    function createBlob(base64EncodedImage) {

        var myString = base64EncodedImage.value.slice(0);

        // This goes into the WordJS library. 
        var byteCharacters = window.atob(myString);

        var byteNumbers = new Array(byteCharacters.length);
        for (var i = 0; i < byteCharacters.length; i++) {
            byteNumbers[i] = byteCharacters.charCodeAt(i);
        }

        var byteArray = new Uint8Array(byteNumbers);

        // THIS IS WHER I HAVE AN ISSUE.
        var blob = new Blob(byteArray);

        // http://stackoverflow.com/questions/18299806/how-to-check-file-mime-type-with-javascript-before-upload, thank you Drakes

        var fileReader = new FileReader();
        var header = "";

        fileReader.onloadend = function (e) {
            var arr = (new Uint8Array(e.target.result)).subarray(0, 4);

            for (var i = 0; i < arr.length; i++) {
                header += arr[i].toString(16);
            }
            console.log(header);

            // Check the file signature against known types

        };
        fileReader.readAsArrayBuffer(blob);

        switch (header) {
        case "89504e47":
            type = "image/png";
            break;
        case "47494638":
            type = "image/gif";
            break;
        case "ffd8ffe0":
        case "ffd8ffe1":
        case "ffd8ffe2":
            type = "image/jpeg";
            break;
        default:
            type = "unknown"; // Or you can use the blob.type as fallback
            break;
        }

    }