There is very few applications which can display AI thumbnails.
But all these applications (including ST Thumbnails Explorer (STTEX) v1.2.2330)
display thumbnails not for all AI files.
It has very simple explanation. Old versions AI (up to v7) don't contain thumbnails at all.
AI v7+ files can be without built-in thumbnails too (e.g. if its are exported from other applications
which do not provide thumbnails creation).
Of course, having Ilustrator 7+ (or other programs which can import these files and save as AI v7+)
these files can be converted to version 7+. However, many individuals and companies have accumulated
very many old AI files, and to convert its manually is very toilfully.
Currently we suggest a partial solution for the problem at least for those who has Illustrator 10-CS3:
ST AI Converter v1.3, the batch converter for all files which can be opened
in Adobe Illustrator 10-CS3 to AI files version 7-CS3.
ST AI Converter (STAICONVERT) is freeware. It does not require any registration.
You may download (staiconv.zip, 102 KB), and try it right now.
Good news. ST Thumbnails Explorer 1.5 Beta 3
can show magnificent (up to 600x600 pixels) thumbnails
for AI and AIT files using CorelDRAW 12+, Corel DESIGNER 12+, and/or Ghostscript. Ghostscript is free for end users,
CorelDRAW 12+ and Corel DESIGNER 12+ are commercial products.
With Ghostscript, STTEX 1.5 Beta 3 can display generated thumbnails for AI files v7-CS5 (the files v10-CS5 must be PDF-compatible).
Nevertheless PDF uncompatible files v10-CS5 and AIT files will not be displayed (the app will display the built-in thumbnails only).
With CorelDRAW 12-X4 or Corel DESIGNER 12+, the app can display all types of AI files up to AI CS2 and AIT v10-CS2 (with CorelDRAW X5,
AI/AIT CS3-CS4 too) already now without any conversion, and save generated thumbnails to the app database
(unfortunately, some files cannot be processed by CorelDRAW/DESIGNER engine).
Please click here for details/last news.
Note.
•.ST AI Converter can be used to convert any files to AI with built-in thumbnails.
Of course, if these files can be opened in Adobe Illustrator.
•.ST AI Converter uses JavaScript engine implemented in Adobe Illustrator 10-CS3.
Therefore, it can be used on all platforms supported by Adobe Illustrator 10-CS3 (e.g. Mac OS or UNIX).
•.The converter was not tested with Adobe Illustrator CS4-CS5.
•.You can use ST EPS Converter
to convert any files to EPS files with embedded previews.
ST AI Converter v1.3 User's Guide
• Requirements
Adobe Illustrator 10+ should be installed.
• Installation scripts
• Unzip the archive staiconv.zip to temp folder and close Illustrator if it's launched.
• Copy all files with extension .js from temp folder to folder
<Illustrator>\Presets\Scripts.
For example: if your copy Adobe Illustrator 10 is installed to
C:\Program Files\Adobe\Illustrator 10, copy the files SaveDocsInVersion7.js,
SaveDocsInVersion8.js, SaveDocsInVersion9.js, and SaveDocsInVersion10.js
from temp folder to C:\Program Files\Adobe\Illustrator 10\Presets\Scripts.
Note. If Adobe Illustrator CS2 is installed to C:\Program Files\Adobe\Illustrator CS2,
copy the files SaveDocsInVersion8.js, SaveDocsInVersion9.js,
SaveDocsInVersion10.js, SaveDocsInVersion11.js, and
SaveDocsInVersion12.js from temp folder to
C:\Program Files\Adobe\Illustrator CS2\Presets\Scripts.
• Copy the file AIVersConvert.aia from temp folder to folder
<Illustrator>\Presets\Action Sets.
Note. If Adobe Illustrator CS/CS2/CS3 is installed
copy the file AIVersConvert.aia
from temp folder to folder <Illustrator CS/CS2/CS3>\Presets\Actions accordingly.
• Script settings
• Launch Adobe Illustrator and open palette Actions
(click in menu: Window|Actions).
• Click expanding button in the right upper corner of the palette and click
Load Actions... 
• In the Open File dialog select AIVersConvert.aia.
It will add the Action Set with the name AIVersConvert to the Actions palette.

• Select in the set AIVersConvert
the appropriate action (ConvertToVersion_7
for conversion to AI v7 with built-in thumbnails, ConvertToVersion_8 for conversion to
AI v8, etc.), and click the triangle at the left. 
Note. JavaScript for Adobe Illustrator CS+ cannot convert
files to AI v7. If you select this action in Adobe Illustrator CS+, the files will be converted to AI v8.
Similarly, JavaScript for Adobe Illustrator 10 cannot convert files to AI v11+.
If you select this action in Adobe Illustrator 10, the files will be converted to AI v10.
Further we assume the files will be converted to version compatible with Illustrator 10.
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• Click expanding button and select in the popup menu Start Recording.

• Click expanding button once again and select Insert Menu Item....

• Click in the main menu File|Scripts|SaveDocsInVersion10
, and click Ok
in the dialog Insert Menu Item.... 
• Click button Stop Playing/Regording
on the bottom this palette.
• Batch conversion
IMPORTANT. If you use the converter with Adobe Illustrator 10,
the transformed files will replace old files without any warnings!
If you use the converter with Adobe Illustrator CS+, the new files will be created with new names and old files don't be changed.
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• Clear folders with processed files from files of types
which don't opened in Illustrator.
• Select the string ConvertToVersion_10 in Actions palette and click Batch...
in expanding button menu. Set up all parameters how its are set on screenshot.
• Click Choose in the Source group of the Batch dialog and select folder with files
which should be converted. For process the subfolders, check Include All Subdirectories option.
Note. If you wish to process at once some thousands files (for example, a CD),
Adobe Illustrator can lose stability. In this case will be reasonable to divide all files on groups for 200-300
files in everyone. Except it, is desirable the Illustrator relaunch after processing each group.
• Click Save As in the Errors group and select folder
for Log Errors.txt file.
• Click Ok for start of the processing. All files from selected folder (and subfolders if you
check Include All Subdirectories option) will be automatically open, processed, saved in the selected
version and closed.
Note. If stability of Illistrator during execution of the script seems to you
unacceptable, try set of the delay after opening a file before its write.
For this purpose before run the script please click expanding button in a right upper corner of the Actions
palette and click Playback Options. In the opened dialog set up Performance parameter
to Pause for: x seconds, where x is the pause in seconds after opening a file prior to the beginning
of its processing by script. We recommend values from 1 to 5 seconds.

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