It would be nice if the readFile basic step could recognize the file's encoding.  (Writing a file with writeFile can store a BOM at the beginning to help in recognizing the character set later, but this must be only a non-default option to preserve backward compatibility with earlier groovy code reading the created files.  This agrees with a rule of thumb "accept liberally, produce conservatively").

There is no need to reinvent the wheel.  Using (and white-listing) groovy.util.CharsetToolkit can make this easy.

https://github.com/groovy/groovy-core/blob/GROOVY_2_4_X/src/main/groovy/util/CharsetToolkit.java#L388

http://docs.groovy-lang.org/latest/html/api/groovy/util/CharsetToolkit.html#getReader()

https://github.com/jenkinsci/workflow-basic-steps-plugin/blob/workflow-basic-steps-2.18/src/main/java/org/jenkinsci/plugins/workflow/steps/ReadFileStep.java