Allow HEAD requests on public files (#26047)

Fixes https://github.com/mattermost/mattermost/issues/25666

```release-note
NONE
```

server/channels/api4/file.go

@@ -41,7 +41,7 @@ func (api *API) InitFile() {
 
 	api.BaseRoutes.Team.Handle("/files/search", api.APISessionRequiredDisableWhenBusy(searchFiles)).Methods("POST")
 
-	api.BaseRoutes.PublicFile.Handle("", api.APIHandler(getPublicFile)).Methods("GET")
+	api.BaseRoutes.PublicFile.Handle("", api.APIHandler(getPublicFile)).Methods("GET", "HEAD")
 }
 
 func parseMultipartRequestHeader(req *http.Request) (boundary string, err error) {

server/channels/api4/file_test.go

@@ -1086,6 +1086,10 @@ func TestGetPublicFile(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, http.StatusOK, resp.StatusCode, "failed to get image with public link")
 
+	resp, err = http.Head(link)
+	require.NoError(t, err)
+	require.Equal(t, http.StatusOK, resp.StatusCode, "failed to respond to HEAD request")
+
 	resp, err = http.Get(link[:strings.LastIndex(link, "?")])
 	require.NoError(t, err)
 	require.Equal(t, http.StatusBadRequest, resp.StatusCode, "should've failed to get image with public link without hash", resp.Status)