SharePoint - Fail to upload certain file types with the Multiple Documents Upload window

When trying to upload .cs, .java, .aspx, or similar files using the Multiple Documents Upload window, the upload fails even though those extensions are allowed in SharePoint. The root cause is IIS, which blocks certain file types by default – particularly files associated with web development (cs, aspx, config, etc.). You also need to allow these extensions in the IIS configuration.

1. Open IIS Manager
2. Select the site corresponding to SharePoint (by default, "SharePoint - 80")
3. Double click on "Request Filtering"
4. Select the desired extension and click "Remove"

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