server: assign a random ID to File's without one during Create

[adamdecaf]

Previously we were only reading out the FileHeader which meant any Batch objects weren't included in the stored file.

Issue: #401

[codecov-io]

Codecov Report

Merging #405 into master will decrease coverage by 0.45%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
- Coverage   85.08%   84.62%   -0.46%     
==========================================
  Files         109      110       +1     
  Lines        6846     8879    +2033     
==========================================
+ Hits         5825     7514    +1689     
- Misses        729     1073     +344     
  Partials      292      292

Impacted Files	Coverage Δ
server/service.go	81.17% <ø> (-4.16%)	⬇️
server/files.go	1.69% <0%> (-0.27%)	⬇️
server/common.go	100% <100%> (ø)
test/ach-adv-read/main.go	55.55% <0%> (-8.09%)	⬇️
test/ach-axt-read/main.go	57.14% <0%> (-8.08%)	⬇️
test/ach-ccd-read/main.go	57.14% <0%> (-8.08%)	⬇️
test/ach-iat-read/main.go	60% <0%> (-8%)	⬇️
test/ach-trc-read/main.go	47.82% <0%> (-7.73%)	⬇️
test/ach-trx-read/main.go	47.82% <0%> (-7.73%)	⬇️
test/ach-xck-read/main.go	47.82% <0%> (-7.73%)	⬇️
... and 99 more

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