Combiner: input_type not specified

When using the combiner to build the output file, the Warning
`WARNING:root:Input formats were only specified for 0 out of 2 files.The last 2 files will remain unformatted.`
will be issued.
As far as I understood the working of combine.py this is because *self.input_format* is not supposed to be None due to the initialization
```python
self.input_format = []
```
on **line 58 in transform.py**
and therefore the if condition
```python
# Adjust length of input_format list
if input_format is None:
fmts = [[] for _ in range(n_inputs)]
```
on **line 392 in combine.py** will never evaluate to True when being called from the *build* function
```python
input_format_list = _build_input_format_list(
input_filepath_list, input_volumes, self.input_format
)
```
( **line 75 in combine.py** )

Is this behavior on purpose?

Start by reading transform.py around line 58 and combine.py around lines 75 and 392, tracing self.input_format through build and _build_input_format_list. Determine the intended handling of an unspecified input format and verify the resulting warning behavior against the issue description. Done means the behavior is intentional or the discrepancy is corrected and verified with the relevant existing tests.

Written by the indexing model from the issue text.