| Hi Timea Antal apologies for the delay. I think something like this might work.
def changes = readFile 'path/to/changelog.md'
And then use this variable in a GString as the input to the AppCenter plugin
appCenter releaseNotes: "${changes}"
Note that this is a GString that support interpolation and not a regular string that doesn't. Also note I haven't had time to try this but I have used this Pipeline step in the past to do similar things. Let me know how you get on  |