JetBook driver: Only use JetBook naming scheme for txt, pdf and fb2 files. Fixes #6638 (Improve the naming scheme for Aluratek Libre (and perhaps Jetbook))

src/calibre/devices/jetbook/driver.py

@@ -50,6 +50,8 @@ class JETBOOK(USBMS):
 
     def filename_callback(self, fname, mi):
         fileext = os.path.splitext(os.path.basename(fname))[1]
+        if fileext.lower() not in ('txt', 'pdf', 'fb2'):
+            return fname
         title = mi.title if mi.title else 'Unknown'
         title = title.replace(' ', '_')
         au = mi.format_authors()