How could we customize/override any functions of default providers like strapi-plugin-upload, say for example, if we want to restrict the image dimension or format and throw error message during upload / save of the file?
Welcome @srirajeshv 
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You can use plugin extensions for it, check out the docs.
This thread might also be related to your question.
If you want to overwrite a upload provider it’s probably best to create your own by forking the one you want to change.
Thank you @MattieBelt for the quick response.
With respect to extensions, we have tired but did not work well. Here is the related issue 8338 reported in GitHub for reference.
So we tried the other option by creating local provider like providers/strapi-plugin-upload and copied the whole bunch of upload plugin source files (~300) into it with package.json pointing to this local provider instead of default upload provider.
Please suggest if there is any other effective way of doing this.
Hi @srirajeshv,
I just posted a answer on that issue, please check it out.
Plugin extension work correctly for the upload plugin.
I saw you figured it out, good luck!