Add lint for input mimetype

[ErikDanielsson]

Check that the input parameter has a mimetype. Should close #1647.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1676 (adc4df6) into dev (2763932) will decrease coverage by 0.08%.
The diff coverage is 65.00%.

@@            Coverage Diff             @@
##              dev    #1676      +/-   ##
==========================================
- Coverage   68.07%   67.99%   -0.09%     
==========================================
  Files          56       56              
  Lines        6588     6609      +21     
==========================================
+ Hits         4485     4494       +9     
- Misses       2103     2115      +12     

Impacted Files	Coverage Δ
nf_core/lint/schema_lint.py	76.66% <62.50%> (-5.16%)	⬇️
nf_core/schema.py	77.39% <66.66%> (-0.27%)	⬇️
nf_core/modules/create.py	79.27% <0.00%> (-1.04%)	⬇️
nf_core/utils.py	80.12% <0.00%> (-0.41%)	⬇️
nf_core/create.py	62.65% <0.00%> (-0.36%)	⬇️
nf_core/__main__.py	52.35% <0.00%> (+0.01%)	⬆️

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[mirpedrol]

Looks good! :)