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:

  1. 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.
  2. 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/",
ppt: "application/",
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";

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