feature #20906 [Validator] Add filenameCharset and filenameCountUnit options to File constraint (alamirault)

This PR was merged into the 7.3 branch.

Discussion
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[Validator] Add `filenameCharset` and `filenameCountUnit` options to `File` constraint

Fix #20703

(Inspired by `Length` options descriptions)

Commits
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e7613b0 [Validator] Add filenameCharset and filenameCountUnit options to File constraint

Author: OskarStark

Diff for: reference/constraints/File.rst

@@ -274,6 +274,31 @@ You can find a list of existing mime types on the `IANA website`_.
 If set, the validator will check that the filename of the underlying file
 doesn't exceed a certain length.
 
+``filenameCountUnit``
+~~~~~~~~~~~~~~~~~~~~~
+
+**type**: ``string`` **default**: ``File::FILENAME_COUNT_BYTES``
+
+The character count unit to use for the filename max length check.
+By default :phpfunction:`strlen` is used, which counts the length of the string in bytes.
+
+Can be one of the following constants of the
+:class:`Symfony\\Component\\Validator\\Constraints\\File` class:
+
+* ``FILENAME_COUNT_BYTES``: Uses :phpfunction:`strlen` counting the length of the
+  string in bytes.
+* ``FILENAME_COUNT_CODEPOINTS``: Uses :phpfunction:`mb_strlen` counting the length
+  of the string in Unicode code points. Simple (multibyte) Unicode characters count
+  as 1 character, while for example ZWJ sequences of composed emojis count as
+  multiple characters.
+* ``FILENAME_COUNT_GRAPHEMES``: Uses :phpfunction:`grapheme_strlen` counting the
+  length of the string in graphemes, i.e. even emojis and ZWJ sequences of composed
+  emojis count as 1 character.
+
+.. versionadded:: 7.3
+
+    The ``filenameCountUnit`` option was introduced in Symfony 7.3.
+
 ``filenameTooLongMessage``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -290,6 +315,35 @@ Parameter                       Description
 ``{{ filename_max_length }}``   Maximum number of characters allowed
 ==============================  ==============================================================
 
+``filenameCharset``
+~~~~~~~~~~~~~~~~~~~
+
+**type**: ``string``  **default**: ``null``
+
+The charset to be used when computing value's filename max length with the
+:phpfunction:`mb_check_encoding` and :phpfunction:`mb_strlen`
+PHP functions.
+
+``filenameCharsetMessage``
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**type**: ``string`` **default**: ``This filename does not match the expected charset.``
+
+The message that will be shown if the value is not using the given `filenameCharsetMessage`_.
+
+You can use the following parameters in this message:
+
+=================  ============================================================
+Parameter          Description
+=================  ============================================================
+``{{ charset }}``  The expected charset
+``{{ name }}``     The current (invalid) value
+=================  ============================================================
+
+.. versionadded:: 7.3
+
+    The ``filenameCharset`` and ``filenameCharsetMessage`` options were introduced in Symfony 7.3.
+
 ``extensionsMessage``
 ~~~~~~~~~~~~~~~~~~~~~