quoting and escaping your JSON correctly, is a imporatnt step and looks like you are missing it.
I might suggest building an array and serializing it into JSON to make it easier to read, or using a HEREDOC or string formatting. If you do continue to concatenate your JSON body string together, dump out the results to see how it is coming together.
The final results of the JSON should look something like
{ "startInfo": { "ReleaseKey":"{{uipath_releaseKey}}", "Strategy":"JobsCount", "JobsCount":1, "InputArguments":"{\"reports_or_other_files\":\"C:\\\\Documents\\\\report.txt\"}" } }
With the InputArguments:
Looks like you are missing some quotes
Might need to double escape your backslashes in the FileListUploaded variable
Missing a colon after the C in the path