Add `Attachments.createWriteForExisting`

This function lets us choose where to write attachment to instead of picking
random name.

app/attachments.js

@@ -56,9 +56,33 @@ exports.createWriterForNew = (root) => {
       throw new TypeError('"arrayBuffer" must be an array buffer');
     }
 
-    const buffer = Buffer.from(arrayBuffer);
     const name = exports.createName();
     const relativePath = exports.getRelativePath(name);
+    return exports.createWriterForExisting(root)({
+      data: arrayBuffer,
+      path: relativePath,
+    });
+  };
+};
+
+//      createWriter :: AttachmentsPath ->
+//                      { data: ArrayBuffer, path: RelativePath } ->
+//                      IO (Promise RelativePath)
+exports.createWriterForExisting = (root) => {
+  if (!isString(root)) {
+    throw new TypeError('"root" must be a path');
+  }
+
+  return async ({ data: arrayBuffer, path: relativePath } = {}) => {
+    if (!isString(relativePath)) {
+      throw new TypeError('"relativePath" must be a path');
+    }
+
+    if (!isArrayBuffer(arrayBuffer)) {
+      throw new TypeError('"arrayBuffer" must be an array buffer');
+    }
+
+    const buffer = Buffer.from(arrayBuffer);
     const absolutePath = path.join(root, relativePath);
     await fse.ensureFile(absolutePath);
     await fse.writeFile(absolutePath, buffer);

test/app/attachments_test.js

@@ -40,6 +40,39 @@ describe('Attachments', () => {
     });
   });
 
+  describe('createWriterForExisting', () => {
+    let tempRootDirectory = null;
+    before(() => {
+      tempRootDirectory = tmp.dirSync().name;
+    });
+
+    after(async () => {
+      await fse.remove(tempRootDirectory);
+    });
+
+    it('should write file to disk on given path and return path', async () => {
+      const input = stringToArrayBuffer('test string');
+      const tempDirectory = path.join(
+        tempRootDirectory,
+        'Attachments_createWriterForExisting'
+      );
+
+      const relativePath = Attachments.getRelativePath(Attachments.createName());
+      const attachment = {
+        path: relativePath,
+        data: input,
+      };
+      const outputPath =
+        await Attachments.createWriterForExisting(tempDirectory)(attachment);
+      const output = await fse.readFile(path.join(tempDirectory, outputPath));
+
+      assert.equal(outputPath, relativePath);
+
+      const inputBuffer = Buffer.from(input);
+      assert.deepEqual(inputBuffer, output);
+    });
+  });
+
   describe('createReader', () => {
     let tempRootDirectory = null;
     before(() => {