Description:
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after mime_content_type function, the locale information is set with the file asked by mime_content_type. Whe need to reset by setlocale() to retrieve the locale information what was before calling mime_content_type().
NB: same issue with finfo_file()

Test script:
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setlocale(LC_ALL, 'fr_FR.UTF-8');

echo iconv("UTF-8", 'ASCII//TRANSLIT//IGNORE', 'bétâ'); // output : beta (OK)

//

$mime=mime_content_type('sheet.xls'); // sheet.xls is not a UTF8 file.

echo iconv("UTF-8", 'ASCII//TRANSLIT//IGNORE', 'bétâ'); // output : b?t? (wrong)

//

$mime=mime_content_type('sheet.xls'); // sheet.xls is not a UTF8 file.

setlocale(LC_ALL, 'fr_FR.UTF-8');

echo iconv("UTF-8", 'ASCII//TRANSLIT//IGNORE', 'bétâ'); // output : beta (OK)