If would be nice if this was already implemented, as lack of encoding makes writing binary files a hassle.

I would like to vote for that fix, because I am using "writeFile" method to overwrite AssemblyInfo.cs on windows, to change version number of application on every build, and right now writeFile is putting into a file '?' sign into first line, and my project is failing during compilation

Rather than adding comments to the effect of “please implement this”, click on the Vote button (and/or offer a tested PR).

PR:

https://github.com/jenkinsci/workflow-basic-steps-plugin/pull/38

Code changed in jenkins
User: Jesse Glick
Path:
src/main/java/org/jenkinsci/plugins/workflow/steps/WriteFileStep.java
src/test/java/org/jenkinsci/plugins/workflow/steps/ReadWriteFileStepTest.java
http://jenkins-ci.org/commit/workflow-basic-steps-plugin/37d4589bc6bbc4cabfc227ebcc8d383470a06942
Log:
Merge pull request #38 from dbut023/jenkins-27094-writefile-encoding

JENKINS-27094 writeFile encoding parameter

Compare: https://github.com/jenkinsci/workflow-basic-steps-plugin/compare/b1b29f6d43dd...37d4589bc6bb

It looks that in the implementation is missing exception for Base64 encoding. By the documentation such text should be decoded first and saved as binary.

It would be fine to fix it. Thanks!

muf support for Base64-encoding would be a separate feature, not covered by this issue.