Ensure mimeType defaults to application/octet-stream given an empty string

[jujaga]

Description

SHOWCASE-3617

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] I have checked that unit tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)

Further comments

[github-actions[bot]]

Coverage Report

Totals
Statements:	61.22% ( 3048 / 4979 )
Methods:	51.15% ( 335 / 655 )
Lines:	68.28% ( 1834 / 2686 )
Branches:	53.66% ( 879 / 1638 )

[codeclimate[bot]]

Code Climate has analyzed commit ea7eadb3 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 68.2% (0.0% change).

View more on Code Climate.