So I build a HTTPS Server wich reads files from SPIFFS. Sometimes the File is just displayed as plaintext instead of HTML

How To Reproduce
Use the code in next Post.
Upload some files to SPIFFS.
Try.

Expected Behavior
I want to display HTML Files.

Actual Behavior
I get the HTML code as plaintext startet with
Content-Type: text/html
Connection: close

ESP32 Module
Please provide specifications of your module

• RAM/PSRAM: non
• Flash Size: 4MB

Software (please complete the following information if applicable)

• IDE and Version: Arduino 1.8.8
• OS: Windows 10/Android 9
• Client used to access the server: Chrome(72.0.3626.121) and Firefox(65.0.1) on Android