Merge pull request #764 from telefonicaid/fgalan-patch-1

FIX improve documentation

docs/usermanual.md

@@ -90,18 +90,23 @@ In this case, sending a single measure, there is possible to send other kinds of
 `application/octet-stream`, not just `application/json`. In case of using `application/octet-stream`, data will be 
 treated as binary data, saved in the attribute maped as hex string. I.E:
 
-For a measure sent to `/iot/json/attrs/attrHex` with value `hello` the resulting attribute value persisten in the context
-broker will be:
+For a measure sent to `POST /iot/json/attrs/attrHex` with content-type: application/octet-stream and binary value
+
+```
+hello
+```
+
+then the resulting attribute sent to ContextBroker:
 
-```json
 {
-    ...
-    "attrHex":"68656c6c6f"
-    ...
+   ...
+   "attrHex": {
+     "value": "68656c6c6f"
+     "type": "<the one used at provisiong time for attrHex attribute>"
+   }
 }
-```
 
-where every group of 2 character (I.E, the first group, `68`) corresponds to a single ASCII character or byte received in 
+Note that every group of 2 character (I.E, the first group, `68`) corresponds to a single ASCII character or byte received in 
 the payload (in this case, the value `0x68` corresponds to `h` in ASCII). You can use one of the multiple tools available 
 online like [this one](https://string-functions.com/string-hex.aspx)
 
@@ -311,6 +316,30 @@ attribute IDs `h` and `t`, then humidity measures are reported this way:
 $ mosquitto_pub -t /json/ABCDEF/id_sen1/attrs/h -m 70 -h <mosquitto_broker> -p <mosquitto_port> -u <user> -P <password>
 ```
 
+In the single measure case, when the published data is not a valid JSON, it is interpreted as binary content. For instance,
+if the following is published to `/json/ABCDEF/id_sen1/attrs/attrHex` topic:
+
+```
+hello
+```
+
+then the resulting attribute sent to ContextBroker:
+
+{
+   ...
+   "attrHex": {
+     "value": "68656c6c6f"
+     "type": "<the one used at provisiong time for attrHex attribute>"
+   }
+}
+
+Note that every group of 2 character (I.E, the first group, `68`) corresponds to a single ASCII character or byte received in 
+the payload (in this case, the value `0x68` corresponds to `h` in ASCII). You can use one of the multiple tools available 
+online like [this one](https://string-functions.com/string-hex.aspx).
+
+Note this works differently that in HTTP transport. In HTTP the JSON vs. binary decission is based on `application/octed-stream` `content-type` header.
+Given that in MQTT we don't have anything equivalent to HTTP headers, we apply the heuristics of checking for JSON format.
+
 #### Configuration retrieval
 
 The protocol offers a mechanism for the devices to retrieve its configuration (or any other value it needs from those