thanks again for the hints, now I’m trying to integrate it into my program.
A new problem: I can’t manage to overwrite my upload plug-in. Can you tell me where the problem is The server does not want to start although I copied the two original files (Upload.js & image-manipulation.js) into it. 
Please see attachments!
Thanks in advance!
E:\CMS\Strapi\project_2021>strapi start
[2020-11-07T13:21:57.268Z] debug ⛔️ Server wasn't able to start properly.
[2020-11-07T13:21:57.271Z] error Error: Cannot find module '../utils/file'
Require stack:
- E:\CMS\Strapi\project_2021\extensions\upload\services\image-manipulation.js
- E:\CMS\Strapi\project_2021\node_modules\strapi\lib\load\load-files.js
- E:\CMS\Strapi\project_2021\node_modules\strapi\lib\core\load-apis.js
- E:\CMS\Strapi\project_2021\node_modules\strapi\lib\core\load-modules.js
- E:\CMS\Strapi\project_2021\node_modules\strapi\lib\Strapi.js
- E:\CMS\Strapi\project_2021\node_modules\strapi\lib\index.js
- E:\CMS\Strapi\project_2021\node_modules\strapi\lib\commands\start.js
- C:\Users\Dingding\AppData\Roaming\npm\node_modules\strapi\bin\strapi.js
at Function.Module._resolveFilename (internal/modules/cjs/loader.js:1065:15)
at Function.Module._load (internal/modules/cjs/loader.js:911:27)
at Module.require (internal/modules/cjs/loader.js:1125:19)
at require (internal/modules/cjs/helpers.js:75:18)
at Object.<anonymous> (E:\CMS\Strapi\project_2021\extensions\upload\services\image-manipulation.js:14:27)
at Module._compile (internal/modules/cjs/loader.js:1236:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1257:10)
at Module.load (internal/modules/cjs/loader.js:1085:32)
at Function.Module._load (internal/modules/cjs/loader.js:950:14)
at Module.require (internal/modules/cjs/loader.js:1125:19)