When generating custom report, leave output format field empty
Description of problem:
When generating a report template that doesn't contain report_render string in template, the Output Format field is greyed out but contains the value of "CSV". This is misleading because the resulting file is a text file, according both to MIME type and to its extension.
Version-Release number of selected component (if applicable):
6.7, feature unavailable in previous versions
Steps to Reproduce:
1. Create a report template that doesn't contain report_render string in template
2. Monitor -> Report Templates -> Generate
Output Format field greyed out, containing "CSV"
Something that is not misleading, e.g. empty value