[BitBucket user: Ben Bloom]
[BitBucket date: 2017-10-26.09:04:59]

###Steps to reproduce:
1. create a dicom file with PatientID tag of length 4 or shorter
2. upload it using curl:

```
#!bash

$ curl -X POST http://user:pw@domain:8042/instances --data-binary @file.dcm
```
Response is:

```
#!json

{
   "HttpError" : "Bad Request",
   "HttpStatus" : 400,
   "Message" : "Bad file format",
   "Method" : "POST",
   "OrthancError" : "Bad file format",
   "OrthancStatus" : 15,
   "Uri" : "/instances"
}
```

Note: Repeating the same steps with PatientID tag length 5 or longer results in success, for example:

```
#!json

{
   "ID" : "9ffb11eb-ade5d153-cd0beb7b-c565e56e-4a04db04",
   "Path" : "/instances/9ffb11eb-ade5d153-cd0beb7b-c565e56e-4a04db04",
   "Status" : "Success"
}

```

### Environment
* Ubuntu 16.04.2
* Orthanc: 1.3.0+dfsg-1+b1