Improve the error message displayed when attaching a file with an allowed Mimetype extension but its content doesn't match the allowed Mimetype

Steps to reproduce

1. Login as Admin
2. Go to Administer Wiki > Content > Attachments Restriction
3. Under "Allowed Mimetypes", insert some file types to be allowed (e.g. text/plain)
4. Under "Blocked Mimetypes", insert some file types to be restricted (e.g. image/*)
5. Save
6. Have an image file with an extension changed to .txt or use the one attached to this issue
7. Login as an user
8. Go to a page and upload this file
9. Observe the received message

Expected results

An error message is displayed informing the user that the content of the file doesn't match the allowed file Mimetypes to be uploaded.

Actual results

A generic error message is displayed:

An error occurred while uploading <filename>.