How to convert files in the computer into short messages and send them by mobile phone?

If it's a text message, you can send it by airmail. Or edit it into a TXT document, put it in your mobile phone, open it with e-book viewing software, copy and paste it into a text message and send it out.

If it is a picture, it is difficult to realize, because the picture supported by mobile phone MMS is very small, only a few tens of KB. If you modify the picture like that, the image will be lost a lot.