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
Fixes #28815 - keep .txt extension for text reports
If a report template does not use report_render macro, the output format
selection is disabled and it's considered a plain text. Mimetype symbol
in this case is text and we use it for suggested file extension. However
for plain text file, it's expected to be .txt. Therefore we need an
exception in the symbol to extension conversion for this case. Other
supported types work fine and we can rely on Mimetype symbols.