plugin download: use cache if dest_path is empty:

if dest_path is not set, the file will be downloaded in cache. A new name will be
generated using a hash of the uri. If the file is already downloded in cache, it won't be
downloaded again (in this case returned progress_id is an empty string).

sat/core/constants.py

@@ -346,6 +346,8 @@ class Const(object):
     ## Files ##
     FILE_TYPE_DIRECTORY = "directory"
     FILE_TYPE_FILE = "file"
+    # when filename can't be found automatically, this one will be used
+    FILE_DEFAULT_NAME = "unnamed"
 
     ## Permissions management ##
     ACCESS_PERM_READ = "read"
@@ -378,10 +380,13 @@ class Const(object):
     ## Misc ##
     SAVEFILE_DATABASE = APP_NAME_FILE + ".db"
     IQ_SET = '/iq[@type="set"]'
-    ENV_PREFIX = "SAT_"  # Prefix used for environment variables
+    # Prefix used for environment variables
+    ENV_PREFIX = "SAT_"
     IGNORE = "ignore"
-    NO_LIMIT = -1  # used in bridge when a integer value is expected
-    DEFAULT_MAX_AGE = 1209600  # default max age of cached files, in seconds
+    # used in bridge when a integer value is expected
+    NO_LIMIT = -1
+    # default max age of cached files, in seconds
+    DEFAULT_MAX_AGE = 3600 * 24 * 14
     HASH_SHA1_EMPTY = "da39a3ee5e6b4b0d3255bfef95601890afd80709"
     STANZA_NAMES = ("iq", "message", "presence")