Prevent selection of unacceptable Content-Type files (#2910)

* Prevent selection of unacceptable Content-Type files

* replace hard code

* media_attachments accept content-types in initial state

app/javascript/mastodon/reducers/media_attachments.js

@@ -0,0 +1,15 @@
+import { STORE_HYDRATE } from '../actions/store';
+import Immutable from 'immutable';
+
+const initialState = Immutable.Map({
+  accept_content_types: [],
+});
+
+export default function meta(state = initialState, action) {
+  switch(action.type) {
+  case STORE_HYDRATE:
+    return state.merge(action.state.get('media_attachments'));
+  default:
+    return state;
+  }
+};