Can't export to CONLL using valueType="url" settings (NER Annotation)

**Describe the bug**
I'm not sure if this is really a bug, or it is the expected behavior of the label-studio. I imported several text files in a row using the time series option with -valueType="url"- set. The task I'm performing is named entity recognition (NER) in word level with custom tags. When I need to export the annotations, the CONLL file generated has the URLs of the imported files instead of words with each tag annotated.

PS: I tried also the saveTextResult="yes" option, but it didn't provide the expected result.

**To Reproduce**
Steps to reproduce the behavior:
1. Import some text files using the valueType="url" option as time series.
2. Annotate some data.
3. Click on export selecting the CONLL2003 option.

**The behavior I got**
```

-DOCSTART- -X- O
/data/upload/3/b5608bfa-61313b676e6be5740da077a0.txt -X- _ O
```

**Expected behavior**

```
-DOCSTART- -X- O
Ouro -X- _ O
Preto -X- _ O
20 -X- _ O
de -X- _ O
fevereiro -X- _ O
de -X- _ O
1866 -X- _ O
Souza -X- _ B-PER
Carvalho -X- _ I-PER
E -X- _ O
```

**Environment (please complete the following information):**
- OS: Ubuntu 20.04
- Label Studio Version 1.4