Commented Unassigned: "load OTP-help file" button for challange-response opens the "file open" dialog instead of YubiChallange [406]

### Description

"load OTP-help file" button for challange-response opens the "file open" dialog instead of YubiChallange. I download some apks from the website here. The error appears since version 0.9.8-pre2 (online version of keepass2android). Version 0.9.7-pre1 and before works. The YubiChallange app is always installed (version 1.0).

### Observed behaviour
the open file dialog (the same as when you select database file) appear if you press the "load OTP-help file" button

### Expected behaviour
YubiChallange app waiting for the yubikey should appear if you press the "load OTP-help file" button

### Reproduce

1. open any file
2. choose the master password type "Password + Challange-Response"
3. Press the button "load OTP-help file"
4. the open file dialog (the same as when you select database file) appear instead of the YubiChallange app waiting for the yubikey

### Environment

* Samsung Galaxy Note Edge
* Android 5.1.1
* YubiChallange version 1.0.
* keepass2android 0.9.8-pre2 (online version of keepass2android) and 0.9.7-pre1 (various other tested)
Comments: Hmm, I had the same issue after some local modifications and finally find that my .xml File with the parameters for challenge/response was missing. After re-adding it, I needed to select the __xml file__ after clicking on "load OTP-help file" once - et voila, YubiChallenge was opened. From now on, it again was opened automatically right after clicking the OTP-file button as expected. So I wonder if the same happened to (some of) the other reporters here, too and if they also just forgot that they need to select the xml once after local modifications (e.g. Update to Android marshmallow shuffling local mount points so that old path to xml gets invalid)...