Fix MIME regex matching for plain files

[boazy]

Summary

• construct the MIME regex search range from mimeType instead of filename
• cover matching with filenames both shorter and longer than the MIME value
• verify that nonmatching MIME values remain rejected

Fixes #370.

Verification

$ Tests/PlainSettingsTests/run.sh
PlainSettingsTests passed

$ xcodebuild -project SourceCodeSyntaxHighlight.xcodeproj \
    -scheme 'Syntax Highlight XPC Service' \
    -configuration Debug \
    -derivedDataPath /tmp/SourceCodeSyntaxHighlight-derived \
    CODE_SIGNING_ALLOWED=NO build
** BUILD SUCCEEDED **