Move writeAttachmentData into Attachments

Encapsulates all attachment operations that require Node.js / Electron APIs,
e.g. file system access.

test/app/attachments_test.js

@@ -0,0 +1,53 @@
+const fse = require('fs-extra');
+const isEqual = require('lodash/isEqual');
+const path = require('path');
+const stringToArrayBuffer = require('string-to-arraybuffer');
+const tmp = require('tmp');
+const { assert } = require('chai');
+
+const Attachments = require('../../app/attachments');
+
+
+const PREFIX_LENGTH = 2;
+const NUM_SEPARATORS = 1;
+const NAME_LENGTH = 64;
+const PATH_LENGTH = PREFIX_LENGTH + NUM_SEPARATORS + NAME_LENGTH;
+
+describe('Attachments', () => {
+  describe('writeData', () => {
+    let TEMPORARY_DIRECTORY = null;
+    before(() => {
+      TEMPORARY_DIRECTORY = tmp.dirSync().name;
+    });
+
+    after(async () => {
+      await fse.remove(TEMPORARY_DIRECTORY);
+    });
+
+    it('should write file to disk and return path', async () => {
+      const input = stringToArrayBuffer('test string');
+      const tempDirectory = path.join(TEMPORARY_DIRECTORY, 'Attachments_writeData');
+
+      const outputPath = await Attachments.writeData(tempDirectory)(input);
+      const output = await fse.readFile(path.join(tempDirectory, outputPath));
+
+      assert.lengthOf(outputPath, PATH_LENGTH);
+
+      const inputBuffer = Buffer.from(input);
+      assert.isTrue(isEqual(inputBuffer, output));
+    });
+
+    describe('createName', () => {
+      it('should return random file name with correct length', () => {
+        assert.lengthOf(Attachments.createName(), NAME_LENGTH);
+      });
+    });
+
+    describe('getRelativePath', () => {
+      it('should return correct path', () => {
+        const name = '608ce3bc536edbf7637a6aeb6040bdfec49349140c0dd43e97c7ce263b15ff7e';
+        assert.lengthOf(Attachments.getRelativePath(name), PATH_LENGTH);
+      });
+    });
+  });
+});