feat(base64-file-converter): infer mime type from base64 signature

src/composable/downloadBase64.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from 'vitest';
+import { getMimeTypeFromBase64 } from './downloadBase64';
+
+describe('downloadBase64', () => {
+  describe('getMimeTypeFromBase64', () => {
+    it('when the base64 string has a data URI, it returns the mime type', () => {
+      expect(getMimeTypeFromBase64({ base64String: 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA' })).to.deep.equal({ mimeType: 'image/png' });
+      expect(getMimeTypeFromBase64({ base64String: 'data:image/jpg;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA' })).to.deep.equal({ mimeType: 'image/jpg' });
+    });
+
+    it('when the base64 string has no data URI, it try to infer the mime type from the signature', () => {
+      // https://en.wikipedia.org/wiki/List_of_file_signatures
+
+      // PNG
+      expect(getMimeTypeFromBase64({ base64String: 'iVBORw0KGgoAAAANSUhEUgAAAAUA' })).to.deep.equal({ mimeType: 'image/png' });
+
+      // GIF
+      expect(getMimeTypeFromBase64({ base64String: 'R0lGODdh' })).to.deep.equal({ mimeType: 'image/gif' });
+      expect(getMimeTypeFromBase64({ base64String: 'R0lGODlh' })).to.deep.equal({ mimeType: 'image/gif' });
+
+      // JPG
+      expect(getMimeTypeFromBase64({ base64String: '/9j/' })).to.deep.equal({ mimeType: 'image/jpg' });
+
+      // PDF
+      expect(getMimeTypeFromBase64({ base64String: 'JVBERi0' })).to.deep.equal({ mimeType: 'application/pdf' });
+    });
+
+    it('when the base64 string has no data URI and no signature, it returns an undefined mimeType', () => {
+      expect(getMimeTypeFromBase64({ base64String: 'JVBERi' })).to.deep.equal({ mimeType: undefined });
+    });
+  });
+});

src/composable/downloadBase64.ts

@@ -1,33 +1,60 @@
 import { extension as getExtensionFromMime } from 'mime-types';
 import type { Ref } from 'vue';
+import _ from 'lodash';
 
-function getFileExtensionFromMime({
-  hasMimeType,
+export { getMimeTypeFromBase64, useDownloadFileFromBase64 };
+
+const commonMimeTypesSignatures = {
+  'JVBERi0': 'application/pdf',
+  'R0lGODdh': 'image/gif',
+  'R0lGODlh': 'image/gif',
+  'iVBORw0KGgo': 'image/png',
+  '/9j/': 'image/jpg',
+};
+
+function getMimeTypeFromBase64({ base64String }: { base64String: string }) {
+  const [,mimeTypeFromBase64] = base64String.match(/data:(.*?);base64/i) ?? [];
+
+  if (mimeTypeFromBase64) {
+    return { mimeType: mimeTypeFromBase64 };
+  }
+
+  const inferredMimeType = _.find(commonMimeTypesSignatures, (_mimeType, signature) => base64String.startsWith(signature));
+
+  if (inferredMimeType) {
+    return { mimeType: inferredMimeType };
+  }
+
+  return { mimeType: undefined };
+}
+
+function getFileExtensionFromMimeType({
+  mimeType,
   defaultExtension = 'txt',
 }: {
-  hasMimeType: string[] | null
+  mimeType: string | undefined
   defaultExtension?: string
 }) {
-  if (hasMimeType) {
-    return getExtensionFromMime(hasMimeType[1]) || defaultExtension;
+  if (mimeType) {
+    return getExtensionFromMime(mimeType) ?? defaultExtension;
   }
 
   return defaultExtension;
 }
 
-export function useDownloadFileFromBase64({ source, filename }: { source: Ref<string>; filename?: string }) {
+function useDownloadFileFromBase64({ source, filename }: { source: Ref<string>; filename?: string }) {
   return {
     download() {
       if (source.value === '') {
         throw new Error('Base64 string is empty');
       }
 
-      const hasMimeType = source.value.match(/data:(.*?);base64/i);
-      const base64String = hasMimeType
+      const { mimeType } = getMimeTypeFromBase64({ base64String: source.value });
+      const base64String = mimeType
         ? source.value
         : `data:text/plain;base64,${source.value}`;
 
-      const cleanFileName = filename ?? `file.${getFileExtensionFromMime({ hasMimeType })}`;
+      const cleanFileName = filename ?? `file.${getFileExtensionFromMimeType({ mimeType })}`;
 
       const a = document.createElement('a');
       a.href = base64String;