You probably gonna find this issue in your manual browsing and spidering phase of your assessment. But also Netsparker will report this issue during your scanning phase.

ZAP

Description

The Content-Type header was either missing or empty.

Solution

Ensure each page is setting the specific and appropriate content-type value for the content being delivered.

References

• https://msdn.microsoft.com/en-us/library/gg622941(v=vs.85).aspx
• Missing Content-Type Header Detected on Target Web Application
• Properly Configuring Server MIME Types