Extract the rules file MIME type into a separate constant

I actually forget why I wanted to do this, I think to explore rule-group
exporting? Seems useful standalone regardless.

Author: pimterry

src/components/mock/mock-page.tsx

@@ -22,7 +22,7 @@ import {
     ItemPath,
     mapRules
 } from '../../model/rules/rules-structure';
-import { serializeRules } from '../../model/rules/rule-serialization';
+import { SERIALIZED_RULES_MIME_TYPE, serializeRules } from '../../model/rules/rule-serialization';
 
 import { clickOnEnter } from '../component-utils';
 import { Button, SecondaryButton } from '../common/inputs';
@@ -347,7 +347,7 @@ class MockPage extends React.Component<MockPageProps> {
             dateFns.format(Date.now(), 'YYYY-MM-DD_HH-mm')
         }.htkrules`;
 
-        saveFile(filename, 'application/htkrules+json;charset=utf-8', rulesetContent);
+        saveFile(filename, SERIALIZED_RULES_MIME_TYPE, rulesetContent);
     }
 }
 

src/model/rules/rule-serialization.ts

@@ -141,4 +141,6 @@ export const deserializeRules = (data: any, args: DeserializationArgs): HtkMockR
     return (
         serializr.deserialize(MockRulesetSchema, migrateRuleData(data), undefined, args)
     ) as HtkMockRuleRoot;
-}
+}
+
+export const SERIALIZED_RULES_MIME_TYPE = 'application/htkrules+json;charset=utf-8';