Change in add_attachment. Allow explicit attachment naming.

[korrosivesec]

Added filename parameter and appropriate checks.

[codecov]

Codecov Report

Merging #178 into master will decrease coverage by 0.05%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
- Coverage   61.05%   60.99%   -0.06%     
==========================================
  Files          17       17              
  Lines        2506     2510       +4     
==========================================
+ Hits         1530     1531       +1     
- Misses        976      979       +3

Impacted Files	Coverage Δ
pymisp/api.py	45.31% <33.33%> (-0.08%)	⬇️

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[coveralls]

Coverage decreased (-0.06%) to 60.996% when pulling 914ac84 on CenturyLinkCIRT:master into 0c763a4 on MISP:master.

[Rafiot]

👍