Determine MIME type from base64

In writing my new Outlook Add-in (Send to Trello), I got stuck on attachments. The first version did not include an attachments option because of two unique problems that compounded each other:

The Office Add-in API no longer provides a Media Type/MIME Type with an attachment request. I am able to get the “blob()” from Office, but other than the file extension there is not a way to determine the type. But sometimes a file does not have an extension, or the extension is wrong, etc.
The Trello API will not let you upload without supplying a MIME type, you cannot just give it a Base64 string as an attachment and let them figure it out.

So, I found out something interesting while researching a workaround. Most every base64 string of a specific file type starts with the same “prolog” of text. Using this, combined with the fallback of the file extensions, I was able to get attachments to work (for the attachment types supported by Trello). So, v1.02 will now include attachments.

Anway, as for the workaround I found, this might be ugly, but wanted to share it anyway:

	/**
	* Returns the data type based on the base64 string
	* @param {String} base64String
	* @param {String} fileName
	* @returns {String}
	*/
	detectMimeType(base64String, fileName) {
	var ext = fileName.substring(fileName.lastIndexOf(".") + 1);
	if (ext === undefined \|\| ext === null \|\| ext === "") ext = "bin";
	ext = ext.toLowerCase();
	const signatures = {
	JVBERi0: "application/pdf",
	R0lGODdh: "image/gif",
	R0lGODlh: "image/gif",
	iVBORw0KGgo: "image/png",
	TU0AK: "image/tiff",
	"/9j/": "image/jpg",
	UEs: "application/vnd.openxmlformats-officedocument.",
	PK: "application/zip",
	};
	for (var s in signatures) {
	if (base64String.indexOf(s) === 0) {
	var x = signatures[s];
	// if an office file format
	if (ext.length > 3 && ext.substring(0, 3) === "ppt") {
	x += "presentationml.presentation";
	} else if (ext.length > 3 && ext.substring(0, 3) === "xls") {
	x += "spreadsheetml.sheet";
	} else if (ext.length > 3 && ext.substring(0, 3) === "doc") {
	x += "wordprocessingml.document";
	}
	// return
	return x;
	}
	}
	// if we are here we can only go off the extensions
	const extensions = {
	xls: "application/vnd.ms-excel",
	ppt: "application/vnd.ms-powerpoint",
	doc: "application/msword",
	xml: "text/xml",
	mpeg: "audio/mpeg",
	mpg: "audio/mpeg",
	txt: "text/plain",
	};
	for (var e in extensions) {
	if (ext.indexOf(e) === 0) {
	var xx = extensions[e];
	return xx;
	}
	}
	// if we are here – not sure what type this is
	return "unknown";
	}