Ensure we drive a useable filename for saving items whose URL does not provide one.

av98.py

@@ -149,6 +149,28 @@ class GeminiItem():
         else:
             return "=> {}\n".format(self.url)
 
+    def derive_filename(self, mime=None):
+        # Simplest option it to use the end of the URL, if there is one.
+        filename = os.path.basename(self.path)
+        if filename:
+            return filename
+
+        # If there's not, try to pretty up the GeminiItem name
+        if self.name:
+            filename = self.name.lower().replace(" ","_")
+        # Otherwise, use something generic.
+        else:
+            filename = "av98_download_" + time.strftime("%Y%m%d%H%M%S")
+
+        # Add an extension
+        if mime == "text/gemini":
+            return filename + ".gmi"
+        elif mime:
+            ext = mimetypes.guess_extension(mime)
+            if ext:
+                return filename + ext
+        return filename + ".file"
+
     @classmethod
     def from_map_line(cls, line, origin_gi):
         assert line.startswith("=>")
@@ -1226,9 +1248,9 @@ Use 'ls -l' to see URLs."""
             gi = self.gi
             saving_current = True
 
-        # Derive filename from current GI's path, if one hasn't been set
+        # Derive a filename if one hasn't been set
         if not filename:
-            filename = os.path.basename(gi.path)
+            filename = gi.derive_filename(self.mime if saving_current else None)
 
         # Check for filename collisions
         if os.path.exists(filename):