Custom reports have file extension ".text"
Description of problem:
Reports that don't use report_render macro have a plaintext format by default when generated. The downloaded file has the extenstion of ".text" which is unusual for text files.
Version-Release number of selected component (if applicable):
Sat 6.7; technically a regression because the extension was .txt in 6.6 but it's not specified what extension the text file should have
Steps to Reproduce:
1. Create a custom report that doesn't have string 'report_render' in template
2. Generate it
A <report>.text file is downloaded
A <report>.txt file is downloaded