Embodiments allow for use of fused files that comprise executable code and content data in a more user-friendly and flexible manner. For example, a fused file can include content data and application logic for editing the content data within the file. Security through the use of digital signatures can be supported, but with at least part of a file recognized as extensible so that the file can be edited without “breaking” the signature. Additionally, a computing device can be configured to utilize a “sandbox” environment so that extensible (and/or other) portions of a fused file do not introduce unacceptable security issues even if the digital signature is valid. In some embodiments, the support of extensibility and sandboxing allows a runtime environment to utilize installer-free operation, which can represent a significant advantage in that fused files can be widely distributed to any device utilizing the runtime environment.