Fix handling double file name extensions

Long names would get truncated at the end, causing problems
including unresolvable file URLs. Example with default settings:
    .package.lst → .package.

Fixes #61

fhost.py

@@ -218,6 +218,8 @@ class File(db.Model):
 
         def get_ext(mime):
             ext = "".join(Path(file_.filename).suffixes[-2:])
+            if len(ext) > app.config["FHOST_MAX_EXT_LENGTH"]:
+                ext = Path(file_.filename).suffixes[-1]
             gmime = mime.split(";")[0]
             guess = guess_extension(gmime)