From MozillaZine Knowledge Base

Thunderbird 2.0 displays large icons for the attachments. You can slightly reduce the size by setting mailnews.attachments.display.largeView to false using the Config editor.

If you don't need the icons you can add the following to the optional userChrome.css file in the chrome subdirectory in in your profile folder. The file does not exist by default, so you may need to create it before you can start adding your preferences.

/* Remove icons in Message pane Attachment view box */
.descriptioncell-icon {
display: none !important;
}

/* Display attachment filenames full length */
.attachmentBox {
-moz-appearance: none !important;
width: auto !important;
}

See also

• Attachment pane height - Thunderbird