Tests and increase consistency for isFileDangerous calls

ts/components/conversation/Message.tsx

@@ -505,7 +505,7 @@ export class Message extends React.Component<Props, State> {
     } else {
       const { fileName, fileSize, contentType } = attachment;
       const extension = getExtension({ contentType, fileName });
-      const isDangerous = isFileDangerous(fileName);
+      const isDangerous = isFileDangerous(fileName || '');
 
       return (
         <div
@@ -738,7 +738,7 @@ export class Message extends React.Component<Props, State> {
       return null;
     }
 
-    const fileName = attachment && attachment.fileName;
+    const fileName = attachment ? attachment.fileName : null;
     const isDangerous = isFileDangerous(fileName || '');
 
     const downloadButton = attachment ? (

ts/test/util/isFileDangerous_test.ts

@@ -0,0 +1,34 @@
+import { assert } from 'chai';
+
+import { isFileDangerous } from '../../util/isFileDangerous';
+
+describe('isFileDangerous', () => {
+  it('returns false for images', () => {
+    assert.strictEqual(isFileDangerous('dog.gif'), false);
+    assert.strictEqual(isFileDangerous('cat.jpg'), false);
+  });
+
+  it('returns false for documents', () => {
+    assert.strictEqual(isFileDangerous('resume.docx'), false);
+    assert.strictEqual(isFileDangerous('price_list.pdf'), false);
+  });
+
+  it('returns true for executable files', () => {
+    assert.strictEqual(isFileDangerous('run.exe'), true);
+    assert.strictEqual(isFileDangerous('install.pif'), true);
+  });
+
+  it('returns false for empty filename', () => {
+    assert.strictEqual(isFileDangerous(''), false);
+  });
+
+  it('returns false for exe at various parts of filename', () => {
+    assert.strictEqual(isFileDangerous('.exemanifesto.txt'), false);
+    assert.strictEqual(isFileDangerous('runexe'), false);
+    assert.strictEqual(isFileDangerous('run_exe'), false);
+  });
+
+  it('returns true for upper-case EXE', () => {
+    assert.strictEqual(isFileDangerous('run.EXE'), true);
+  });
+});