Replace regex usage for MIME parsing

[gabriel-vasile]

MIME types can have multiple optional parameters, eg:
video/webm; codecs="w/e codec"; charset="binary"
For these cases, regex will fail to drop all params from the MIME type.

This commit replaces the usage of regex for getting the "type/subtype" with mime.ParseMediaType.

[codecov-commenter]

Codecov Report

Merging #17831 (e9eb764) into main (371ebce) will increase coverage by 0.00%.
The diff coverage is 45.45%.

@@           Coverage Diff           @@
##             main   #17831   +/-   ##
=======================================
  Coverage   45.54%   45.54%           
=======================================
  Files         808      808           
  Lines       90049    90056    +7     
=======================================
+ Hits        41009    41018    +9     
+ Misses      42487    42484    -3     
- Partials     6553     6554    +1     

Impacted Files	Coverage Δ
modules/upload/upload.go	73.21% <25.00%> (-4.15%)	⬇️
routers/web/web.go	90.34% <57.14%> (-0.33%)	⬇️
modules/charset/charset.go	71.71% <0.00%> (-4.05%)	⬇️
modules/queue/queue_channel.go	95.00% <0.00%> (-1.67%)	⬇️
services/pull/pull.go	41.78% <0.00%> (ø)
routers/api/v1/repo/pull.go	29.35% <0.00%> (+0.51%)	⬆️
modules/queue/queue_bytefifo.go	59.88% <0.00%> (+0.59%)	⬆️
modules/queue/queue_disk_channel.go	62.72% <0.00%> (+1.77%)	⬆️
modules/queue/workerpool.go	50.00% <0.00%> (+2.29%)	⬆️

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[silverwind]

Could you add a testcase or two here? Should be pretty simple.

[gabriel-vasile]

I tried to think of a regression test for this change but I don't think there is any.
All MIME types returned by http.DetectContentType are processed correctly with the regex because they don't have multiple optional params nor they have any weird syntax like text/plain; param="weird;value".

Honestly, the only value this PR has is if sometime in the future http.DetectContentType is updated to return one of those weird MIME types for which the regex will fail.