Introduces the file path check

Introduces wpcf7_is_file_path_in_content_dir() to support the use of the UPLOADS constant.

This vulnerability affects the following application versions:

• Contact Form 7 4.7
• Contact Form 7 4.8
• Contact Form 7 4.8.1
• Contact Form 7 4.9
• Contact Form 7 4.9.1
• Contact Form 7 4.9.2
• Contact Form 7 5.0
• Contact Form 7 5.0.1
• Contact Form 7 5.0.2
• Contact Form 7 5.0.3
• Contact Form 7 5.0.4