Openai: use message attachment as GPT context

[brainexe]

Usecases:

• upload a log file with a strange error
• upload a .csv file to gather some statistics
• upload a code file to refactor

[codecov-commenter]

Codecov Report

Merging #490 (9a09e95) into master (ef59d0f) will increase coverage by 0.19%.
Report is 15 commits behind head on master.
The diff coverage is 73.63%.

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@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
+ Coverage   82.18%   82.38%   +0.19%     
==========================================
  Files         130      133       +3     
  Lines        5934     6074     +140     
==========================================
+ Hits         4877     5004     +127     
- Misses        933      940       +7     
- Partials      124      130       +6     

Files	Coverage Δ
bot/bot_handle.go	100.00% <100.00%> (ø)
bot/bot_init.go	62.35% <100.00%> (+0.44%)	⬆️
bot/matcher/result.go	100.00% <ø> (+64.28%)	⬆️
bot/stats/stats.go	76.00% <100.00%> (ø)
bot/storage/memory.go	93.18% <100.00%> (ø)
bot/util/lock.go	100.00% <100.00%> (ø)
command/jenkins/build_watcher.go	38.94% <100.00%> (ø)
command/jenkins/client/client.go	100.00% <100.00%> (ø)
command/jenkins/client/start_build.go	87.05% <100.00%> (ø)
command/jenkins/commands.go	100.00% <ø> (ø)
... and 14 more

... and 2 files with indirect coverage changes

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