What you are trying to do should work exactly as you have described it.

I tried the same thing on my CICS TS 4.2 system and it worked as expected.

I used this simple piece of HTML:

 <form method="POST" action="http://host:port/javaform" enctype="multipart/form-data">
 <input type="file" id="exa" name="infile" accept="image/*"  >
 <input type="submit" name="Test" value="Test">
 </form>

In CICS this drove the following java program which reads the formfield value and returns the input image file back to the browser;

     public static void main(String[] args) {
         Task t = Task.getTask();
         if ( t == null )
             System.err.println("CICSWeb: Can't get Task");
         else
         {
             try
             {
             System.out.println("CICSWeb: Started");
             HttpRequest request = HttpRequest.getHttpRequestInstance();
             byte[] infile = request.getFormFieldData("infile");
             System.out.println("CICSWeb: input file field length is " + infile.length);
             
             System.out.println("CICSWeb: generating response");
             HttpResponse response = new HttpResponse();
             response.setMediaType("image/jpg");
             response.setNoServerConvert();
             response.sendFrom(infile);
             
             System.out.println("CICSWeb: Completed");
             }
             catch (Exception e)
             {
                 e.printStackTrace();
                 System.out.println("CICSWeb: Unexpected exception " + e.getMessage());
             }
         }
     }
 

If this is still failing for you then you could try an EXEC CICS WEB RECEIVE to get all the data CICS has received into your application. That would show if the right data has been sent in. You could also use CEDF to debug each JCICS command. A CICS trace would also show what is going on (you would need SO and WB trace components set to 1-2 and others at 1).