MM-23408: Added channel ID check for Plugin API UploadFile (#14138)

* Added 2 checks for app/file.go
- Check if channel id exist
- Check if user has permission to the upload file to the channel

Also added translations for 2 errors defined in app/file.go

* fixed 1 failing test that was linked with UploadFile

* Fixed small issue, according to the review.

* missed 1 review item. Just updated the code for it.

* fix 1 failing test, assuming that the file upload is required. ignoring nouser idea.

Added the translation for english for 2 newly defined errors.

* removed new line

* trying to fix the translation issue. Added the missing translations from master.

* as per discussion, we need to revert the check for user channel permission. So reverted it.

* Update app/file_test.go

Co-Authored-By: Alejandro García Montoro <alejandro.garciamontoro@gmail.com>

* Update i18n/en.json

Co-Authored-By: Alejandro García Montoro <alejandro.garciamontoro@gmail.com>

* Update file.go

Move the check to the top of the method.

* go fmt

Co-authored-by: mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Alejandro García Montoro <alejandro.garciamontoro@gmail.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

app/file.go

@@ -434,8 +434,13 @@ func (a *App) UploadFiles(teamId string, channelId string, userId string, files
 
 // UploadFile uploads a single file in form of a completely constructed byte array for a channel.
 func (a *App) UploadFile(data []byte, channelId string, filename string) (*model.FileInfo, *model.AppError) {
-	info, _, appError := a.DoUploadFileExpectModification(time.Now(), "noteam", channelId, "nouser", filename, data)
+	_, err := a.GetChannel(channelId)
+	if err != nil && channelId != "" {
+		return nil, model.NewAppError("UploadFile", "api.file.upload_file.incorrect_channelId.app_error",
+			map[string]interface{}{"channelId": channelId}, "", http.StatusBadRequest)
+	}
 
+	info, _, appError := a.DoUploadFileExpectModification(time.Now(), "noteam", channelId, "nouser", filename, data)
 	if appError != nil {
 		return nil, appError
 	}