There seems to be an issue with the way media type webhook and script parameters are functioning. 

Steps to reproduce:

Part 1:

1. Go to Alerts -> Media types
2. Create new media type
3. Fill in the data:
   1. Name: test
   2. Type: Webhook
   3. Parameters: Leave the default parameters unchanged
   4. Script: test
4. Press "Add" to create media type.
5. Open the created media type "test"
6. Observe: All data is correct.
7. Change the data:
   1. Type: Script
   2. Script name: test
   3. Do not add script parameters!
8. Update the media type.
9. Open the edited media type "test"
10.  Observe: There are 4 script parameters added.

Result:

Part 2:

1. Remove all of the parameters;
2. Update "test" media type;
3. Open "test" media type;
4. Observe: Three parameters are still added. 

Part 3:

1. Remove all 3 parameters;
2. Add new parameter with value "test";
3. Update "test" media type
4. Open "test" media type;
5. Observe: Three parameters are still added - two empty + previously added parameter "test".

Part 4:

1. Edit the "test" media type and change data:
   1. Type: Webhook
   2. Script: "test"
2. Do not  change the automatically added parameters!
3. Update media type
4. Open "test" media type and observe: No parameters are added.

Expected:
Media type and script parameters are not "mixed" together when media type Type is changed from Webhook to script and the other way around.

Note: This affects version 6.4 and master. Looks like the issue can be reproduced after commit b1002a8c5c7.