Add plugin API for UploadFile method (#9684)

* Add plugin API for UploadFile method

* Add minimum server version documentation to plugin API UploadFile function

* Reorganize some imports

plugin/api.go

@@ -309,6 +309,11 @@ type API interface {
 	// Minimum server version: 5.6
 	GetEmojiImage(emojiId string) ([]byte, string, *model.AppError)
 
+	// UploadFile will upload a file to a channel using a multipart request, to be later attached to a post.
+	//
+	// Minimum server version: 5.6
+	UploadFile(data []byte, channelId string, filename string) (*model.FileInfo, *model.AppError)
+
 	// KVSet will store a key-value pair, unique per plugin.
 	KVSet(key string, value []byte) *model.AppError