Bug report: Structure with file extension .txt fails with bug report

## Automated report

_This issue was created with the app's automated bug reporting feature.
Attached to this issue is the full traceback as well as an environment
fingerprint that contains information about the operating system as well as all
installed libraries._

## Additional comments (optional):

I uploaded a file with extension .extxyz.txt (by mistake actually) but I get a bug report.
I should probably only get a nice warning saying that the format is chosen from the extension (if this is the case) and that .txt is now valid, and to upload a different file.

## Attachments

Traceback

```python-traceback
~/.local/lib/python3.9/site-packages/aiidalab_widgets_base/structures.py in _on_file_upload(self, change)
425 """When file upload button is pressed."""
426 for fname, item in change["new"].items():
--> 427 self.structure = self._read_structure(fname, item["content"])
428 self.file_upload.value.clear()
429 break

~/.local/lib/python3.9/site-packages/aiidalab_widgets_base/structures.py in _read_structure(self, fname, content)
447 structures = get_ase_from_file(temp_file.name, format="cif")
448 else:
--> 449 structures = get_ase_from_file(temp_file.name)
450 except ValueError as e:
451 self._status_message.message = f"""

~/.local/lib/python3.9/site-packages/aiidalab_widgets_base/utils/__init__.py in get_ase_from_file(fname, file_format)
47 traj = ase.io.read(fname, format=file_format, index=":", store_tags=True)
48 else:
---> 49 traj = ase.io.read(fname, format=file_format, index=":")
50 if not traj:
51 raise ValueError(f"Could not read any information from the file {fname}")

/opt/conda/lib/python3.9/site-packages/ase/io/formats.py in read(filename, index, format, parallel, do_not_split_by_at_sign, **kwargs)
793 format = format or filetype(filename, read=isinstance(filename, str))
794
--> 795 io = get_ioformat(format)
796 if isinstance(index, (slice, str)):
797 return list(_iread(filename, index, format, io, parallel=parallel,

/opt/conda/lib/python3.9/site-packages/ase/io/formats.py in get_ioformat(name)
296 """Return ioformat object or raise appropriate error."""
297 if name not in ioformats:
--> 298 raise UnknownFileTypeError(name)
299 fmt = ioformats[name]
300 # Make sure module is importable, since this could also raise an error.

UnknownFileTypeError: txt
```

Environment fingerprint

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time of error.**