What is the issue with CodeIgniter 4 when uploading a PDF-file due to an invalid mime type?

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The issue with CodeIgniter 4 when uploading a PDF-file due to an invalid mime type is that CodeIgniter 4 has strict validation on file uploads and only accepts certain mime types. If the uploaded file has a mime type that is not on the list of accepted types, CodeIgniter 4 will reject the file and return an error message. This can be problematic when trying to upload files such as PDFs that have a mime type that is not recognized by CodeIgniter 4. To resolve this issue, the developer can add the mime type of PDF files to the accepted list in CodeIgniter 4's validation rules.