The files you have been given are the files that are created as part of the build of your project and the resultant executable files. There is no source included here.
In a BB OS Build, regardless of the development environment used, the Java source files will all have the suffix .java, and the assets (images etc.) will have a suffix appropriate to the asset (like .png). If you don't see these file suffixes, then you have not been given the actual source. You should be able to view the java files using Notepad, the other files will open in an appropriate application (like paint).
To get the complete source, you should just ask the full 'project' files for your development. This will be a directory with a number of sub directories. The actual names used and the structure will depend on the development tool. If your developer is using Eclipse, then the two important directories are called src for source and res (resources) for the assets. If they are using another development environment, then the directories might have different names. So you should ask them what development environment they are using too.
Two other points:
1) If you are paying for this development and wish to review the code, but are not familiar with Java, then I would recommend that you pay someone to review the code who has knowledge of BB Java. There are two reasons for this:
(a) you will not be able to form a judgement on the appropriateness of the code without some understanding of Java, and
(b) you will not understand if the correct BB Java approaches have been used.
You need to be cautious about this, because programmers will always find fault in other developer's code. The question is how significant the faults are.
2) Some developers might be wary of giving source to their client while some payment is outstanding.