Speed up loading of large files when guessing format

[astrofrog]

This adds a shortcut if loading a large file that isn't a FITS file. Previously astropy would try and load the whole file as a header.

[codecov]

Codecov Report

Merging #1920 into master will decrease coverage by 0.04%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #1920      +/-   ##
==========================================
- Coverage   85.64%   85.59%   -0.05%     
==========================================
  Files         236      236              
  Lines       21044    21072      +28     
==========================================
+ Hits        18023    18037      +14     
- Misses       3021     3035      +14

Impacted Files	Coverage Δ
glue/core/data_factories/fits.py	87.68% <90%> (-1.39%)	⬇️
glue/app/qt/mdi_area.py	60% <0%> (-6.67%)	⬇️

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