The help section for "Properties File Path" states: Gives a file path of a properties file. The file format must be the standard Java property file format.

I have a property file that is in the Java property file format (it was written out by the store method of java.util.Properties), but the #, !, =, : are left escaped when envinject places the variable in the jenkins job.

The linked page for Java property file format gives an example of:
website = http\://en.wikipedia.org/
In the example above, website would be a key, and its corresponding value would be http://en.wikipedia.org/
I used this example in a property file that envInject used to place variables into the job and the result is:
website http\://en.wikipedia.org/