Now passing strings to mimetypes.guess_extension.

Also changing the default for images to .img (from None).

Also, whatever was originally implementing mime2extension did not include
the leading dot that mimetypes includes.  We are doing away with that, too.

This is supposed to address bug #552.

pyvo/dal/mimetype.py

@@ -46,11 +46,11 @@ def mime2extension(mimetype, default=None):
     if not mimetype:
         return default
 
-    if isinstance(mimetype, str):
-        mimetype = mimetype.encode('utf-8')
+    if isinstance(mimetype, bytes):
+        mimetype = mimetype.decode('utf-8')
 
     ext = mimetypes.guess_extension(mimetype, strict=False)
-    return ext
+    return ext[1:]
 
 
 def mime_object_maker(url, mimetype, *, session=None):

pyvo/dal/sia.py

@@ -903,7 +903,7 @@ def suggest_dataset_basename(self):
             out = re.sub(r'\s+', '_', out.strip())
         return out
 
-    def suggest_extension(self, *, default=None):
+    def suggest_extension(self, *, default='img'):
         """
         returns a recommended filename extension for the dataset described
         by this record.  Typically, this would look at the column describing