BUG: provide filename when reading a non-archived file from mail

[kapalex]

Please read the notes below and replace them with the description of you pull request

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• OpenStack Wiki on GitCommitMessages

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Description

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[codecov-commenter]

Codecov Report

Merging #2021 (5994d26) into develop (b6f807e) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##           develop    #2021   +/-   ##
========================================
  Coverage    75.99%   76.00%           
========================================
  Files          423      423           
  Lines        22778    22778           
  Branches      3022     3022           
========================================
+ Hits         17311    17312    +1     
  Misses        4758     4758           
+ Partials       709      708    -1     

Impacted Files	Coverage Δ
...elmq/bots/collectors/mail/collector_mail_attach.py	76.47% <0.00%> (ø)
intelmq/bots/experts/ripe/expert.py	77.55% <0.00%> (+1.02%)	⬆️

[ghost]

Thanks! Also cherry-picked for maintenance.
I added tests for this case in 8a8107e