If you are getting this error for valid files, try using an external tool for detecting the MIME type. The guide shows how to disable/enable mimetype checking on uploads.
File type validation using extension and MIME types
A reliable security check is an important task if you allow strangers upload files to your server. Ajax uploader allows you checks the file name extension of the uploaded file against a hard-coded list of allowed file name extensions and rejects all other types of files. Why happen if people rename their file extensions (rename .exe file extension as .jpg) and uploading them to your server?
Faking extensions is used extensively to mislead people as the the file type. This can be used to malware of various kinds. It can also lead to security bugs. If myfile.txt would be validated for security as a text file, and later by content inspection is run as an executable, you may end up with an infected machine.
Ajax uploader allows developers check the file name or MIME type of the file to make sure that users have uploaded a file that you want to accept.
Enable mimetype checking on file uploads
<ValidateOption EnableMimetypeChecking="true" />
Disable mimetype checking on file uploads
<ValidateOption EnableMimetypeChecking="false" />