mimetype not alwats present as key

[wizardofzos]

Try CBT 888 for instance.... one of the members get's parsed like this:

"DUMPFILE": {
 "ttr": 97,
 "alias": True,
 "halfwords": 34,
 "notes": 1,
 "parms": "b"\\ x00\\ x00\\ x00\\ x00\\....",
 "ispf": False
},

Then the mimetype isn't there.....

Posting as an issue as I might try and create a PR that will always fill all the 'dict-keys' so post processing doesn't have to to things like this..

if 'mimetype' in xmijson['file'][dsnam]['members'][m]:
    mt = xmijson['file'][dsnam]['members'][m]['mimetype']
else:
    mt = 'application/octet-stream' # force it :)

The same is true (as seen in JSON above) for datatype and extension