2 files changed, 66 insertions, 1 deletions
diff --git a/packages/gitbook/src/models/__tests__/book.js b/packages/gitbook/src/models/__tests__/book.js
new file mode 100644
index 0000000..2c09546
--- /dev/null
+++ b/packages/gitbook/src/models/__tests__/book.js
@@ -0,0 +1,65 @@
+const Book = require('../book');
+const File = require('../file');
+const Readme = require('../readme');
+const Summary = require('../summary');
+const Glossary = require('../glossary');
+
+function createMockBook(def) {
+    return new Book({
+        readme: new Readme({
+            file: new File({ path: def.readme })
+        }),
+
+        summary: new Summary({
+            file: new File({ path: def.summary })
+        }),
+
+        glossary: new Glossary({
+            file: new File({ path: def.glossary })
+        })
+    });
+}
+
+describe('Book', () => {
+    describe('getDefaultExt', () => {
+        it('must infer from README', () => {
+            const book = createMockBook({
+                readme: 'README.adoc',
+                summary: 'SUMMARY.md',
+                glossary: 'GLOSSARY.md'
+            });
+
+            expect(book.getDefaultExt()).toBe('.adoc');
+        });
+
+        it('must infer from SUMMARY', () => {
+            const book = createMockBook({
+                readme: '',
+                summary: 'SUMMARY.adoc',
+                glossary: 'GLOSSARY.md'
+            });
+
+            expect(book.getDefaultExt()).toBe('.adoc');
+        });
+
+        it('must infer from GLOSSARY', () => {
+            const book = createMockBook({
+                readme: '',
+                summary: '',
+                glossary: 'GLOSSARY.adoc'
+            });
+
+            expect(book.getDefaultExt()).toBe('.adoc');
+        });
+
+        it('must default to .md', () => {
+            const book = createMockBook({
+                readme: '',
+                summary: '',
+                glossary: ''
+            });
+
+            expect(book.getDefaultExt()).toBe('.md');
+        });
+    });
+});
diff --git a/packages/gitbook/src/models/book.js b/packages/gitbook/src/models/book.js
index ca445ea..1366e95 100644
--- a/packages/gitbook/src/models/book.js
+++ b/packages/gitbook/src/models/book.js
@@ -259,7 +259,7 @@ class Book extends Record(DEFAULTS) {
         const exts = clues.map((clue) => {
             const file = clue.getFile();
             if (file.exists()) {
-                return file.getParser().getExtensions().first();
+                return file.getParser().FILE_EXTENSIONS[0];
             } else {
                 return null;
             }