Modified Base64 (when /,= and + are replaced) is safe to create names but does not guarantee reverse transformation due to case insensitivity of many file systems and urls.
Base64 is case sensitive, so it will not guarantee 1-to-1 mapping in cases of case insensitive file systems (all Windows files systems, ignoring POSIX subsystem cases). Most urls also case insensitive preventing 1-to-1 mapping.
I would use Base32 in this case – you’ll get names a bit longer, but Base32 encoded values are 100% safe for file/uri usage without replacing any characters and guarantees 1-to-1 mapping even in cases of insensitive environment (FAT/Win32 NTFS access).
Unfortunately there is usually no built-in support for this encoding in frameworks. On other hand code is relatively simple to write yourself or find online.