make attachment specs more realistic

the sent params to not match the ones that are actually sent when running the server.
change them to mirror the real world better

`:params` in tests

```
[1] pry(#<WorkPackagesController>)> params
=> {"attachments"=>{"file"=>{"file"=>"test1.test", "description"=>""}},
 "project_id"=>"15",
 "controller"=>"work_packages",
 "action"=>"create"}
```

`:params` when really uploading stuff

```
[1] pry(#<WorkPackagesController>)> params
=> ...
 "attachments"=>
  {"1"=>
    {"file"=>
      #<ActionDispatch::Http::UploadedFile:0x007fb6c297d238
       @content_type="text/plain",
       @headers=
        "Content-Disposition: form-data; name=\"attachments[1][file]\"; filename=\"karsten.txt\"\r\nContent-Type: text/plain\r\n",
       @original_filename="karsten.txt",
       @tempfile=
        #<File:/var/folders/vy/8c6gt6bx2cv8v3ynzyby9mw80000gp/T/RackMultipart20141218-56776-4qhw4r>>,
     "description"=>""}},
```

with this pr:

`:params` in tests

```
[1] pry(#<WorkPackagesController>)> params
=> {"attachments"=>
  {"1"=>
    {"file"=>
      #<ActionDispatch::Http::UploadedFile:0x007fc81c041eb0
       @content_type="text/plain",
       @headers=nil,
       @original_filename=nil,
       @tempfile=
        #<File:/Users/Martin/code/opf/openproject-attachments/test/fixtures/files/testfile.txt>>,
     "description"=>""}},
 "project_id"=>"2",
 "controller"=>"work_packages",
 "action"=>"create"}
```