• Using the Build Step: "Inject environment variables"
  with following parameters:
  "Properties File Path:" E:\test\my_env.properties
  "Properties Content:" LIBS_ALL=x

• Checking result with: "Execute Windows batch command"
  Command: set

Result:

• Correct setting of environment variables set with "Properties Content" (e.g. LIBS_ALL)
• No environment variables set by using "Properties File Path"

Relating Console output:
...
[EnvInject] - Injecting as environment variables the properties file path 'E: / test / my_env.properties'
[EnvInject] - Injecting as environment variables the properties content
'LIBS_ALL=x'

...
...\workspace>set
...
LIBS_ALL=x
...

Note:
Please consider the spaces in the path of the console output ('E: / test / my_env.properties')!

The same "my_env.properties" file was tested using "Jenkins Environment File Plugin"

• Build Triggers: Set environment variables through a file
  -> Using the "Jenkins Environment File Plugin", the environment variables are set correctly!