TYPO3 Core

I can confirm this (6.2.4, Linux & Win).

When renaming a file all spaces are converted to underscores. It's not possible to rename "My_Image" back to "My Image". If you try to, you will get error 'Destination "%s" existed already!'

White spaces in file pathes are not nice but should be accepted.

Typo3 version 6.2.(3) replaces all characters that are not within [.a-zA-Z0-9_-] with a "_". This might be helpful in most cases but one should definitely be able to use whitespaces when renaming files manually, maybe with a "force" option.

When renaming (local) files there is a call to:
\TYPO3\CMS\Core\Resource\FolderInterface\LocalDriver::sanitizeFileName()

This function preg_replaces all forbidden characters with a "_", including the whitespace.

The matching pattern is stored in
\TYPO3\CMS\Core\Resource\FolderInterface\LocalDriver::UNSAFE_FILENAME_CHARACTER_EXPRESSION

It's a hex pattern and is set to '\\x00-\\x2C\\/\\x3A-\\x3F\\x5B-\\x60\\x7B-\\xBF'
The first range x00-x2C includes the whitespace (HexCode = x20).

I propose to exclude the whitespace from the pattern like this (but I'm not that experienced in regular expressions):
\\x00-\\x19\\x21-\\x2C\\/\\x3A-\\x3F\\x5B-\\x60\\x7B-\\xBF

Maybe a parameter should be added to the sanitizeFileName function to enable or disable whitespaces filtering.

Alternatively, the call to filename "sanitization" could be omitted in \TYPO3\CMS\Core\Resource\ResourceStorage::renameFile()