Make H5Wasm demo more permissive

[axelboc]

• Allow files with any extensions to be dropped - fix Allow loading of files with non-standard extensions in h5wasm demo #1126
• Allow files that don't start with the HDF5 magic number (called the HDF5 File Signature in the spec), as per the HDF5 spec https://web.ics.purdue.edu/~aai/HDF5/html/H5.format.html

[loichuder]

Note that the magic number is also called File Signature in the format spec on HDFGroup's website: https://portal.hdfgroup.org/display/HDF5/File+Format+Specification