1. The file is a gzip archive:
```
% gunzip -vl Parcel
method crc date time compressed uncompressed ratio uncompressed_name
defla 1564e121 Apr 14 05:27 439516 759456 42.1% Parcel
```

Ungzip: `gunzip -c Parcel > parcel_content`

2. Look at the text file inside. Notice that it has 2 types of content (apart from multiparts): text and png images:
```
% cat parcel_content | grep Content-Type | sort | uniq
Content-Type: image/png
Content-Type: multipart/mixed; boundary="===============0000136577486898548=="
...
Content-Type: text/plain; charset="us-ascii"
```

Let's ignore text and focuse on images.

3. Extract images:

```
% cat extractor.py
import sys
import base64

if __name__ == '__main__':
with open(sys.argv[1], 'rt') as fin:
lines = [line for line in fin]
i = 0
file_count = 0
seen_b64_contents = set()
while i < len(lines):
if 'Content-Type: image/png' in lines[i]:
assert 'MIME-Version: 1.0' in lines[i + 1]
assert 'Content-Transfer-Encoding: base64' in lines[i + 2]
assert '' == lines[i + 3].strip()
i += 4
b64_content = ''
while lines[i].strip() != '':
b64_content += lines[i]
i += 1

if b64_content not in seen_b64_contents:
seen_b64_contents.add(b64_content)
with open('img_{}.png'.format(1 + file_count), 'wb') as fout:
content = base64.decodebytes(b64_content.encode())
fout.write(content)
file_count += 1
print('Extracted image #{}'.format(file_count))
i += 1

% python3 extractor.py parcel_content
Extracted image #1
...
Extracted image #25
```

4. Notice some images have letters. Import them to any visual editor (I used draw.io), drop those without letters, reorder and get the flag:

Flag is `RS{Im_doing_a_v1rtual_puzzl3}`.