### What
Introduces an additional `mimeType` validation based on the actual file
data to ensure the uploaded file matches the allowed `mimeTypes` defined
in the upload config.
### Why?
The current validation relies on the file extension, which can be easily
manipulated. For example, if only PDFs are allowed, a JPEG renamed to
`image.pdf` would bypass the check and be accepted. This change prevents
such cases by verifying the true MIME type.
### How?
Performs a secondary validation using the file’s binary data (buffer),
providing a more reliable MIME type check.
Fixes#12905
5695d22a46