Given a file parameter with file location as RAC_INFO_REAL.xlsx
And node is a linux slave
When build the job with no file selected
Then the job workspace become an empty file (not a directory) causing pipeline failure

build.log

stage('upload') {
	steps {
		// delete workspace
		cleanWs()
		script {
			// handle file parameters in pipeline (JENKINS-27413)
			// https://stackoverflow.com/questions/59468464/fetching-uploaded-files-in-jenkins
			def file_in_workspace = unstashParam 'RAC_INFO_REAL.xlsx'
			sh "mv '${file_in_workspace}' RAC_INFO_REAL.xlsx"
                }
	}
}

I am using like this in my pipeline.

       def file_in_workspace = unstashParam "test_data.xlsx"
       sh "cat ${file_in_workspace} > test_data.xlsx"

It's not clearly documented, but that construct is unsafe. See https://jenkins.io/blog/2017/02/01/pipeline-scalability-best-practice/ , specifically:

@NonCPS functions can use a much broader subset of the Groovy language, such as iterators and functional features, which makes them more terse and fast to write.

And while working around that, pay attention to:

@NonCPS functions should not use Pipeline steps internally, however you can store the result of a Pipeline step to a variable and use it that as the input to a @NonCPS function.

Gotcha: It’s not guaranteed that use of a step will generate an error (there is an open RFE to implement that), but you should not rely on that behavior. You may see improper handling of exceptions, in particular.

It's annoying, but it's there.

Any idea why I would get a FileNotFoundException when I trigge a Job using unstashParam from an upstream job?
Not sure if I'm doing something wrong or if it's a bug.

For details, see: https://stackoverflow.com/questions/71292365/jenkins-pipeline-unstashparam-nosuchfileexception-in-downstream-job