Extract target_extname to allow 'html:HTML:EmbedImages'

[yhirano55]

fix#27

When the target format was 'html:HTML:EmbededImages', the current implementation of the extension extraction logic generated an incorrect file extension of 'html:HTML'.

To address this issue, we have revised the extension extraction logic to correctly identify and generate the appropriate file extension for the target format.

[yhirano55]

✔️ All specs are passed:

$ bundle exec rspec
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Libreconv::Converter
#convert
converts a xlsx to pdf
converts a xls to pdf
converts a file to the specified target html
converts a pptx to pdf
converts a ppt to pdf
converts a file to the specified target html with EmbedImages options
converts a doc to pdf
converts a docx to pdf
Libreconv::Converter
#check_valid_url
returns NIL for non-exists url
returns URI for valid url
raises HTTP error when requesting failed
raises error for invalid URI
returns false for non-HTTP url
Libreconv::Converter
#build_file_uri
converts a file path to file URI
builds file URI from a path with special characters
Libreconv::Converter
.new
raises error if soffice command does not exists
raises error if source does not exists
:soffice_command
returns the command found in path
returns the user specified command path
Libreconv::Converter
#convert
raises error when URL cannot be loaded
converts a source URL to pdf
Libreconv
has a version number
.convert
converts an office file to the specified target pdf
Finished in 49.46 seconds (files took 0.56574 seconds to load)
23 examples, 0 failures
Randomized with seed 8690