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FOA, code looks clear and clean, well indented and formatted. Thumbs up!!

Now, to the comments:

• Some of the variable names might have more significant names: is can be changed to input_stream, for instance.
• File size: it looks like you try to get the file size out of the stream by fetching the stream until reaching its end. I'd change that to an explicit call to a function provider by the hosting OS' API (see stat), or by a library that implements it, like boost.
• Extract file size calculation to a function, to achieve code reuse.
• Base64 format should convert a byte array (or a binary stream) to an ASCII string. It has nothing to do in Bitmap format in specific. I'd recommend you create some test cases using a C++ unit testing framework (there are some) and find the bug, if such exists.

Good luck!