Add MIME type support

[engival]

Add universal MIME type support

New Features

• --type MIMETYPE for any MIME type (e.g., --type "image/png", --type "text/html")
• --html shortcut for --type "text/html"
• --query option to list available clipboard targets
• Smart text/binary distinction with proper fallback handling

Examples

Binary data (images):

# Copy PNG image to clipboard
cat /etc/kde/xdg/slackware_logo.png | xsel --type 'image/png' --input -b

# Now paste directly into GIMP

HTML content:
# Copy rich HTML to clipboard
echo '<b>Hello <i>World</i></b>' | xsel --html --input -b

# Now paste into Gmail compose window

Diagnostic to query clipboard contents:
# See what's available in clipboard
xsel --query -b
# Output might show: TARGETS, image/png, STRING, UTF8_STRING, etc.

Technical Details

• Binary data fix: Saves input length instead of relying on strlen() (but only for mime type, doesn't change old behavior)
• Smart fallbacks: Text-like MIME types (text/*, application/json, etc.) provide STRING/UTF8_STRING fallbacks for compatibility -- not a comprehensive list!
• Binary barrier: Binary MIME types block text atom registration to prevent unwanted conversions

Compatibility

• All existing functionality preserved
• MIME type is only added when requested

Tested with GIMP (PNG images), Gmail (HTML), and various text editors.

[kratusch]

This is great—looks like I can finally drop xclip now that xsel supports MIME types and is also prioritized by default in neovim due to known issues with xclip (See neovim#20862 and neovim-docs).

I tested copying image/png data to the clipboard, and it works as expected—it can be pasted into other applications.

One request: could MIME types support also be extended to output operations? For example:

xsel --type 'image/png' --output --clipboard

Currently, this results in the following error:

X Error of failed request:  BadAtom (invalid Atom parameter)
  Major opcode of failed request:  24 (X_ConvertSelection)
  Atom id in failed request:  0x1c0
  Serial number of failed request:  25
  Current serial number in output stream:  26