Fix unresolved variable mimeType

[pdesgarets]

Screenshot currently is broken (local variable 'mimeType' referenced before assignment). This PR fixes it.

[codecov-io]

Codecov Report

Merging #3 into dev will not change coverage.
The diff coverage is 40%.

@@           Coverage Diff           @@
##              dev       #3   +/-   ##
=======================================
  Coverage   78.96%   78.96%           
=======================================
  Files          19       19           
  Lines        1659     1659           
=======================================
  Hits         1310     1310           
  Misses        349      349

Impacted Files	Coverage Δ
pyppeteer/page.py	77.77% <40%> (ø)	⬆️

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[miyakogi]

Good catch!
Thank you!